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A Living Theory Catalogue for Fast Radio Bursts

E. Plattsa,∗, A. Weltmana, A. Waltersb,c, S. P. Tendulkard, J.E.B. Gordina, S. Kandhaia

aHigh Energy Physics, Cosmology & Astrophysics Theory (HEPCAT) group, Department of Mathematics andApplied Mathematics, University of Cape Town, Rondebosch, 7700, South Africa

bAstrophysics & Cosmology Research Unit, School of Chemistry and Physics, University of KwaZulu-Natal,Durban, 4000, South Africa

cNAOC-UKZN Computational Astrophysics Centre (NUCAC), University of KwaZulu-Natal, Durban, 4000, SouthAfrica

dDepartment of Physics & McGill Space Institute, McGill University, 3600 University Street, Montreal QC, H3A2T8, Canada

Abstract

At present, we have almost as many theories to explain Fast Radio Bursts as we have Fast RadioBursts observed. This landscape will be changing rapidly with CHIME/FRB, recently commis-sioned in Canada, and HIRAX, under construction in South Africa. This is an opportune timeto review existing theories and their observational consequences, allowing us to efficiently curtailviable astrophysical models as more data becomes available. In this article we provide a currentlyup to date catalogue of the numerous and varied theories proposed for Fast Radio Bursts so far. Wealso launched an online evolving repository for the use and benefit of the community to dynamicallyupdate our theoretical knowledge and discuss constraints and uses of Fast Radio Bursts.

Keywords: Fast Radio Bursts, transients, neutron stars, black holes

∗Corresponding authorEmail addresses: pltemm002@myuct.ac.za (E. Platts), amanda.weltman@uct.ac.za (A. Weltman),

waltersa@ukzn.ac.za (A. Walters), shriharsh@physics.mcgill.ca (S. P. Tendulkar), grdjak001@myuct.ac.za(J.E.B. Gordin), kndsul001@myuct.ac.za (S. Kandhai)

Preprint submitted to Elsevier June 10, 2019

Contents

1 Introduction 4

2 Basic Observational Constraints 6

2.1 Dispersion Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2 Polarization and Rotation Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 Observed Counterparts / Possible Counterparts . . . . . . . . . . . . . . . . . . . . . 7

2.3.1 FRB 150418 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3.2 FRB 131104 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3.3 FRB 121102 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.4 The Curious Case of FRB 121102 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Model Ingredients 8

3.1 Emission Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.1.1 Bremsstrahlung Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.1.2 Atomic Electron Transition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.1.3 Synchrotron Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.1.4 Curvature Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.1.5 Undulator Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.1.6 Inverse Compton Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2 Generating Coherence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.2.1 Bunched Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.2.2 Masers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.2.3 Dicke’s Superradiance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Progenitor Theories 11

4.1 Compact Object Mergers / Interactions . . . . . . . . . . . . . . . . . . . . . . . . . 114.1.1 Neutron Star–Neutron Star Mergers / Interactions . . . . . . . . . . . . . . . 114.1.2 Neutron Star–Supernova Interactions . . . . . . . . . . . . . . . . . . . . . . . 124.1.3 Neutron Star–White Dwarf Mergers . . . . . . . . . . . . . . . . . . . . . . . 124.1.4 Binary White Dwarf Merger . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.1.5 White Dwarf–Black Hole Mergers . . . . . . . . . . . . . . . . . . . . . . . . . 134.1.6 Neutron Star–Black Hole Mergers . . . . . . . . . . . . . . . . . . . . . . . . 134.1.7 Pulsar–Black Hole Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . 144.1.8 Kerr-Newman–Black Hole Interactions . . . . . . . . . . . . . . . . . . . . . . 14

4.2 Collapse of Compact Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.2.1 Supramassive Neutron Star to Kerr-Newman Black Hole . . . . . . . . . . . . 154.2.2 Neutron Star to Quark Star . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.2.3 Dark Matter Induced Neutron Star Collapse . . . . . . . . . . . . . . . . . . 154.2.4 Collapse of Strange Star Crust . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.3 Supernovae Remnants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.3.1 Giant Pulses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.3.2 Giant Flares in Magnetars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.3.3 Ejecta Penetration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.4 Active Galactic Nuclei . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.4.1 AGN Jet Interacting with Cavitons . . . . . . . . . . . . . . . . . . . . . . . . 20

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4.4.2 Kerr Black Hole Interacting with AGN . . . . . . . . . . . . . . . . . . . . . . 204.4.3 Strange Star Interacting with AGN . . . . . . . . . . . . . . . . . . . . . . . . 214.4.4 AGN-like Wandering Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.5 Collisions and Close Encounters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.5.1 Neutron Stars and Small Bodies . . . . . . . . . . . . . . . . . . . . . . . . . 214.5.2 Collisions Between Neutron Stars and Primordial Black Holes . . . . . . . . . 224.5.3 Interactions Between Axions and Compact Bodies . . . . . . . . . . . . . . . 22

4.6 Other Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.6.1 Starquake-Induced Repeaters . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.6.2 Variable Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.6.3 Lightning in Pulsars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.6.4 Wandering Pulsar Beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.6.5 Tiny Electromagnetic Explosions . . . . . . . . . . . . . . . . . . . . . . . . . 254.6.6 White Hole Explosions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.6.7 Neutron Star Combing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.6.8 Neutral Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264.6.9 Superconducting Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264.6.10 Dicke’s Superradiance in Galaxies . . . . . . . . . . . . . . . . . . . . . . . . 264.6.11 Alien Light Sails . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.7 Theories That Have Been Ruled Out . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.7.1 Stellar Coronae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.7.2 Annihilating Mini Black Holes . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5 Conclusion 27

5.1 Future Observational Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.1.1 Astrophysical Formation Channels . . . . . . . . . . . . . . . . . . . . . . . . 285.1.2 Emission Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

6 Acknowledgements 28

7 Tabulated Summary 29

8 Acronyms 31

9 References 33

3

1. Introduction

A little over a decade after their discovery (Lorimer et al., 2007), Fast Radio Bursts (FRBs) remainan enigmatic class of radio transients. FRBs are characterized by one or multiple very bright (∼ Jy)and very brief (∼ ms) bursts of radio photons, and have been detected at frequencies rangingbetween 400 MHz− 8 GHz by a number of ground-based radio telescopes. Importantly, the arrivaltime of the frequency components is dispersed, precisely going as ∆t ∼ ν−2, which is consistentwith the propagation of a radio wave through cold plasma (Lorimer et al., 2007; Dennison, 2014;Caleb et al., 2016). While some early speculation considered FRBs to be of galactic origin (Keaneet al., 2012; Burke-Spolaor and Bannister, 2014; Bannister and Madsen, 2014; Maoz et al., 2015),current consensus is that their excessively large dispersion measure (DM) (see Section 2.1), highgalactic latitude, apparent isotropy over the sky (Champion et al., 2016), and lack of HII regionsor other sources of excess DM indicate an extragalactic (Keane et al., 2012; Xu and Han, 2015; Xuand Han, 2015; Cordes and Wasserman, 2016; Connor et al., 2016) or cosmological (Dolag et al.,2015; Kulkarni et al., 2014; Katz, 2016d; Caleb et al., 2016; Vedantham et al., 2016a; Cao et al.,2017; Niino, 2018) origin. This consensus is supported by the identification of the host galaxy ofFRB 121102 at redshift z=0.1932 (Chatterjee et al., 2017; Tendulkar et al., 2017).

One of the central challenges facing theoretical model builders is finding a physical mechanism withwhich one can explain the vast amount of energy radiated over such short timescales. If one assumesisotropic emission, the extreme brightness indicates that some beamed, coherent emission processis required (Thornton et al., 2013), and the brevity of the signals suggests the source is extremelycompact (Thornton et al., 2013). Compounding the model building challenges, the characteristicproperties of FRBs appear to be heterogeneous. Where measurements have been possible, FRBshave been observed to have circular (Ravi et al., 2015; Petroff et al., 2015a; Caleb et al., 2018)and/or linear (Petroff et al., 2015b; Masui et al., 2015; Ravi et al., 2016; Michilli et al., 2018; Gajjaret al., 2018) polarizations, as well as some that seem unpolarized (although this may be due toextremely high Faraday rotation (Michilli et al., 2018)). The pulse profiles of FRBs also differ: twohave double or triple peaks (Champion et al., 2016; Farah et al., 2018), while the rest have onlysingle peaks. Many FRBs have now shown complex microstructure and features at timescales of10s of microseconds (Farah et al., 2018) (and Hessels et al., in preparation). Even more baffling isthat only two FRBs, FRB 121102 and FRB 180814.J0422+73 have been observed to repeat (Spitleret al., 2016; Scholz et al., 2017; Gajjar et al., 2018; Spitler et al., 2018; CHIME/FRB Collaborationet al., 2019), with modulating pulse shapes and no apparent periodicity (see Section 2.4). SomeFRBs have been monitored for up to hundreds of hours with no indication of repetition (Lorimeret al., 2007; Petroff et al., 2015a; Ravi et al., 2016; Petroff et al., 2017; Bhandari et al., 2018;Shannon et al., 2010), and while this may imply there are two different classes of FRB (repeatersand non-repeaters) (Palaniswamy et al., 2018; Michilli et al., 2018), there may just be a large rangein repetition rates (Caleb et al., 2019). Further, the observed repetition or lack thereof can bestrongly affected by interstellar scintillations (Cordes and Rickett, 1998) or plasma lensing (Cordeset al., 2017; Main et al., 2018; Spitler et al., 2018). FRB 121102 was found to have a rotationmeasure (RM) 400 times larger than any other known FRB (Michilli et al., 2018; Gajjar et al.,2018), but previous FRBs may have had high RMs that were simply not detectable (Petroff et al.,2015a; Keane et al., 2016). There is currently no consensus on the matter.

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There have only been 541 FRB detections subsequent to the Lorimer burst in 20072, but the non-detection of FRBs can provide some insight. For example, one can constrain event rates (Siemionet al., 2012; Wayth et al., 2012; Trott et al., 2013a,b; Tingay et al., 2015; Karastergiou et al.,2015; Burke-Spolaor et al., 2016; Rowlinson et al., 2016; Amiri et al., 2017; Surnis et al., 2017),spectral indices (Tingay et al., 2015; Karastergiou et al., 2015; Burke-Spolaor et al., 2016), andsurface densities of transients (Rowlinson et al., 2016). Arguably, the most illuminating detectionto date is (the repeating) FRB 121102 (Spitler et al., 2016). This is the only event to have beensuccessfully associated with a host galaxy: a low-metallicity star-forming dwarf galaxy at a redshiftof z = 0.1932 (Chatterjee et al., 2017; Tendulkar et al., 2017; Bassa et al., 2017). This confirmsthat (at least some) FRBs propagate over extragalactic or cosmological distances.

Apart from being interesting in and of themselves, FRBs could prove to be very useful cosmologicaland astrophysical probes. There are already a number of proposals FRBs to study fundamentalcosmological parameters (Zhou et al., 2014; Yang and Zhang, 2016; Walters et al., 2018; Yu andWang, 2017; Li et al., 2018b; Zitrin and Eichler, 2018; Wei et al., 2018), extragalactic magneticfields (Vazza et al., In Press), properties of the intergalactic medium (IGM) (Deng and Zhang,2014; Zheng et al., 2014; Macquart et al., 2015; Akahori et al., 2016; Fujita et al., 2017; Shull andDanforth, 2018; Ravi, 2018), dark matter distribution (Gao et al., 2014; Muoz et al., 2016; Wangand Wang, 2018), photon mass (Wu et al., 2016; Bonetti et al., 2016, 2017; Wei et al., 2017; Shaoand Zhang, 2017; Wei and Wu, 2018), the Equivalence Principle (Wei et al., 2015; Tingay andKaplan, 2016; Zhang, 2016c; Yu et al., 2018; Bertolami and Landim, 2018; Yu and Wang, 2018),the cosmic web (Ravi et al., 2016), cosmic microwave background (CMB) optical length (Fialkovand Loeb, 2016), and superconducting cosmic strings (Yu et al., 2014; Cao and Yu, 2018). Theefficacy of some of these approaches will, however, rely on the ability to model and remove the FRBhost galaxy contribution to the observed DM.

The sparsity of observational data has led to the publication of a plethora of progenitor theoriesin recent years. These can be broadly broken into two groups: those which appeal to astrophysicalmechanisms for which there is some empirical evidence (such as plasma physics); and those whichappeal to aspects of (mathematical) physics. The latter are more speculative and/or lack anyempirical evidence, but nonetheless may be well-motivated from a theoretical standpoint. Whilethe former class may be more appealing to some, here we take an agnostic stance and summarizeboth sets of possibilities.

We are aware of recent FRB review articles (Katz, 2016b; Petroff, 2017; Romero et al., 2016; Popovet al., 2018; Katz, 2018; Popov et al., In Press), which have some overlap with this work. However,here we compile a complete catalogue of the FRB progenitor theories published in literature to date.We split the theories into different physical classes, and focus on possible observable signatures andcounterparts, with the aim of helping to guide future observing strategies and expedite the processof ruling out theories.

Since the theoretical ideas regarding FRBs are rapidly evolving, we have launched the contentsof this review as an evolving online catalogue at http://frbtheorycat.org. This online wiki isintended to be a starting point for further discussions amongst theorists, observers and instrument

1Correct on the date of submission, almost doubled in the week of submission (Shannon et al., 2010)!2See the online FRB catalogue (Petroff et al., 2016), found at www.frbcat.org, for an up-to-date list.

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builders regarding FRB theories, observational constraints and future tests. Readers are encouragedto participate in the wiki by signing up and contributing to the discussion. Over the next few years,this wiki will record the zeitgeist of the FRB community. As the wiki stores all changes made tothe website, it will also serve as a record of how this scientific community converged on the correctanswers.

The organization of this paper is as follows: in Section 2, we summarize the current state of FRBobservations and in Section 3, we discuss the basic physical processes that are invoked in FRBtheories. These sections are intended as an introduction to the FRB field for new graduate-levelresearchers. In Section 4, we summarize the proposed FRB theories and their predictions. For thereader’s convenience, we offer a tabulated summary as well as a glossary of the acronyms in Section7.

2. Basic Observational Constraints

2.1. Dispersion Measures

One of the primary observables of an FRB is the delay in arrival time between different frequencycomponents of the burst. This delay is proportional to the dispersion measure DM =

∫nedl, i.e.

the column density of free electrons along the line of sight from the source to the observer. For anextragalactic FRB, the DM is expected to be the sum of contributions from the Milky Way’s disc(Yao et al., 2017) and halo (Dolag et al., 2015), the IGM (McQuinn, 2014; Akahori et al., 2016), thedisc and halo of the host galaxy (Xu and Han, 2015; Yang et al., 2017; Tendulkar et al., 2017), andthe local environment of the FRB progenitor (Lyutikov et al., 2016; Piro, 2016; Yang and Zhang,2017), while the DM of a galactic FRB must be entirely accounted for by the Milky Way and/orthe local environment of the progenitor (Connor et al., 2016). Of the FRBs so far observed at highgalactic latitude, most have large DM that is difficult to account for with gas in the Milky Way,and so point toward and extragalactic origin (Xu and Han, 2015). The presence of excess DMcontributed by Hα filaments or HII regions has also been ruled out for most FRBs.

Since all but one FRB (see Section 2.4) consist of a single pulse with no observable counterparts,they have not been sufficiently localized on sky to be associated with any astronomical object,and thus their distance is unknown. However, if one assumes that the dominant contribution theDM is due to diffuse gas in the IGM, one can estimate their distance/redshift. These estimatessuggest a cosmological origin (Dolag et al., 2015; Kulkarni et al., 2014; Katz, 2016d; Caleb et al.,2016; Vedantham et al., 2016a; Cao et al., 2017; Niino, 2018). As other contributions to the DMare additive, however, these estimates are always upper limits (Cordes et al., 2016). If the sourceresides in a galaxy and/or is surrounded by ejecta/nebula, the DM will increase from this localcontribution (Keane et al., 2012; Xu and Han, 2015; Xu and Han, 2015; Cordes and Wasserman,2016; Connor et al., 2016).

2.2. Polarization and Rotation Measures

Linearly polarized light passing through a plasma with a magnetic field component along the di-rection of travel undergoes Faraday rotation—where the plane of polarization rotates around thedirection of travel by an angle proportional to the rotation measure and wavelength squared. The

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RM is defined as∫B||nedl, where B|| is the component of magnetic field parallel to the direction

of travel.

Contributions to the RM can be from the Milky Way’s disc (Oppermann et al., 2012) and/or halo,the intergalactic medium (IGM) (Akahori et al., 2016), the disc and/or halo of the host galaxy, orthe local environment (Connor et al., 2016; Piro, 2016; Lyutikov et al., 2016; Michilli et al., 2018).For the handful of FRBs that have been successfully measured, polarizations are observed to becircular (Petroff et al., 2015a; Caleb et al., 2018) and/or linear (Masui et al., 2015; Ravi et al., 2016;Petroff et al., 2017; Michilli et al., 2018; Gajjar et al., 2018). The RMs have also been varied—someare very high (Masui et al., 2015; Caleb et al., 2018; Michilli et al., 2018) and others are very low(Keane et al., 2016; Petroff et al., 2017; Caleb et al., 2018). The polarizations and RMs thereforeserve as a useful probe of the magneto-ionic environment associated with the FRB. Together withthe DM, one can also use the RM to approximate the mean magnetic field strength between asource and an observer.

2.3. Observed Counterparts / Possible Counterparts

The presence of a signal unequivocally associated with an FRB will break the viability of cer-tain theories. Here we present the FRBs for which potential counterparts have been suggested ordetected.

2.3.1. FRB 150418

In 2016, the first possible FRB counterpart was claimed (Keane et al., 2016): a ∼ 6 day radiotransient following FRB 150418 with emission consistent with a short gamma ray burst (sGRB) af-terglow (Keane et al., 2016; Zhang, 2016b). The signal lacks significant high-energy γ-ray afterglow(Abdalla et al., 2017), and has shown no apparent optical variation (Niino et al., 2018). Though thesource was initially thought to be a fading transient, follow up observations (Williams and Berger,2016) show the emission is consistent with an active galactic nuclei (AGN) (Akiyama and John-son, 2016; Williams and Berger, 2016; Vedantham et al., 2016b; Bassa et al., 2016; Giroletti et al.,2016; Niino et al., 2018), however the AGN may not be associated with FRB 150418 (Williams andBerger, 2016; Giroletti et al., 2016). The chance of coincidence is arguably low (Johnston et al.,2017; Li and Zhang, 2016), however the general consensus is that the events are unrelated.

2.3.2. FRB 131104

A relatively long duration (∼ 100s) γ-ray transient, Swift J0644.5–5111 (DeLaunay et al., 2016),was observed shortly after FRB 131104 (Ravi et al., 2015). The emission is consistent with a gammaray burst (GRB). The presence an expected multi-wavelength afterglow was not observed (Shannonand Ravi, 2017), but arguably may not be observable (Gao and Zhang, 2017). A high-energy (GeV)γ-ray afterglow is not affected by environment density, but searches for such signals have yieldednull results (Xi et al., 2017). The lack of counterparts suggests that FRB 131104 is not associatedwith the GRB-like emission. Further, the signal to noise ratio of the detection is very low, and assuch it is generally agreed that the emission is not an FRB counterpart.

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2.3.3. FRB 121102

To date, only one reported FRB has unequivocally been associated with counterparts. The repeatingFRB 121102 (see Section 2.4) was monitored by the Very Large Array in a fast-dump recordingmode (Law et al., 2015), facilitating the discovery of a persistent radio and optical counterpart, andallowing for its localization on a sub-arcsecond scale (Chatterjee et al., 2017; Tendulkar et al., 2017).Balmer and [OIII] emission lines appear in the opical spectrum that are similar to extreme emission linegalaxies (Tendulkar et al., 2017). Further observations by Arecibo and the European VLBI Network (EVN)found the signals lie within 40 pc of each other (Marcote et al., 2017), providing strong evidence that theFRB and radio counterpart are indeed associated. In light of this, FRB 120112’s physical origin was swiftlyidentified: a low-metallicity star-forming dwarf galaxy at redshift 0.19273(8) (Tendulkar et al., 2017; Bassaet al., 2017), with the persistent radio counterpart offset from the center of the galaxy (Tendulkar et al.,2017).

New evidence suggests that this dwarf galaxy host may not be generic—the recent localizations of non-repeating FRBs to the outskirts of elliptical galaxies (Bannister et al in preparation; private communication)suggests that FRBs come from heterogeneous environments, possibly suggesting multiple astrophysicalformation channels.

2.4. The Curious Case of FRB 121102

As seen in the previous section, FRB 121102 is unique, being the first FRB known to repeat and the onlyone to have had its location accurately determined. As such, one can study its local environment andnarrow down theories describing its source. Though FRB 121102 may be representative of an FRB typedistinct from the non-repeaters, it provides the first solid insights into the nature of these strange sources.

The FRB emission has a relatively small DM and the bursts are consistently 100% linearly polarizedwith very high and variable Faraday RMs: RMsrc = +1.46 × 105 rad/m2 and +1.33 × 105 rad/m2 atepochs separated by seven months with narrow temporal structure (< 30µs) (Michilli et al., 2018). Thesemeasurements have constrained the size of the emitting region to be . 10 km in diameter (Michilli et al.,2018). Making FRB 121102 even more unique is that the RMs are 500 times larger than those reportedfor any other FRB (Michilli et al., 2018; Gajjar et al., 2018). Other signatures are that the polarizationposition angle for FRB 121102 appears to stay constant through each burst and does not display the usualS-shaped curve that is seen in pulsar pulses (Michilli et al., 2018). The intrinsic polarization position angledoesn’t change significantly from burst to burst either, with a timescale of months.

3. Model Ingredients

Different models employ different radiation mechanisms to produce the observed radio characteristics ofFRBs, and have a range of corresponding signatures. The mechanisms themselves are based on radiationproduction, and fall broadly into one of several classes. For the reader’s edification and comfort, belowwe will present each of the overarching radiation mechanisms that astrophysical sources can produce.Additionally, mechanisms to generate the required coherence are given. See (Rybicki and Lightman, 1979)for a comprehensive study.

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3.1. Emission Mechanisms

3.1.1. Bremsstrahlung Radiation

Translated as “braking radiation” or “deceleration radiation”, Bremsstrahlung occurs when the path ofa free electron is bent as it passes a free ion. This deflection causes the electron to decelerate and emitfree-free radiation.

3.1.2. Atomic Electron Transition

Electrons bound to an atom have discrete energy levels. An electron can either absorb electromagnetic(EM) radiation and jump to a higher energy state, or emit radiation when dropping to a lower energy state.

3.1.3. Synchrotron Radiation

A charged particle moving in a magnetic field with some component of velocity perpendicular to the fieldwill be forced on a helical trajectory, and emit radiation. The energy and intensity of the emission isdetermined by the curvature of the charged particle’s trajectory. For charged particles that spiral tightlyaround magnetic field lines, the main contribution to the radiation is from the helix curvature. Suchradiation is called: synchrotron (for ultra-relativistic particles), cyclotron (for non-relativistic particles), orgyro-synchrotron (for moderately relativistic particles), and covers a broad range of the electromagneticspectrum.

3.1.4. Curvature Radiation

When the helical path of a charged particle in a magnetic field is stretched out, the curvature of oscillationsaround the field lines becomes negligible compared to the curvature of the magnetic field line itself, whichbecomes the dominant contributor to emission. Ultra-relativistic particles that move in a strong magneticfield, for instance, can quickly lose their rotational energy through synchrotron emission, and begin to emitcurvature radiation as they stream along the field lines.

3.1.5. Undulator Radiation

When electrons pass through an alternating electric field, they are made to oscillate. The radiation issynchrotron for relativistic electrons and cylotron for non-relativistic electrons.

3.1.6. Inverse Compton Scattering

When photons are scattered by ultra-energetic electrons, the resultant energy transfer can create photons atsignificantly higher energies—X-rays or γ-rays. The spectral energy distribution of the photons is dependenton the original spectrum of the photons, the momentum distribution of the electrons, and the optical depthof interaction. In the case of curvature radiation, the photon momenta are closely aligned with the electronmomenta and experience little inverse Compton scattering. However, in the case of synchrotron radiation,the misalignment of photons and electron velocities causes high levels of inverse Compton scattering, andthus X- and γ-rays are expected counterparts of the radio emission.

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3.2. Generating Coherence

The high energetics and short timescales of FRBs have led many authors to presume coherent radiation intheir progenitor theories. Coherent radiation is emitted by: bunched particles accelerating along EM fieldlines (also known as an “antenna” mechanism (Buschauer and Benford, 1976)); by masers, in which theemission from simultaneous electron phase transitions is amplified; or by a Dicke’s superradiance (DSR),in which particles collectively undergo an atomic transition.

3.2.1. Bunched Particles

When particles accelerating along magnetic field lines are bunched into groups such that they have the sameoscillatory phase, the radiation emitted is coherent. There are various astrophysical bunching mechanismspossible, described below.

3.2.1.1. Two-Stream Plasma Instability. Consider a plasma in a steady-state force-free magnetosphere withtwo or more positron and electron streams moving on curved field lines. The velocities of these streamswill differ if the net charge density of the plasma varies. The streaming between oppositely charged regionsof plasma excites unstable electrostatic plasma waves. The waves trap the particles, forming bunches ofelectrons and positrons that are accelerated along the magnetic field lines. Streaming can be induced whenparticles are injected into a plasma.

3.2.1.2. Magnetic Reconnection. A violent disturbance can cause magnetic field lines from different do-mains to snap and then splice together. Such an event causes particles to bunch in different regions of themagnetic field lines. The relaxation of the magnetic field to a lower energy state accelerates these bunchesto relativistic speeds, creating coherent radiation.

3.2.1.3. Magnetic Braking. When ionized material from a differentially rotating body is captured by thebody’s magnetic field and is propelled into space, the body spins down; an effect known as magnetic braking.The ejected bunches of particles are accelerated radially along the magnetic field lines and emit high-energyphotons.

3.2.2. Masers

Here we limit our discussion to synchrotron masers 3. Consider the following scenario: if an incoming photonis absorbed by an excited electron, the electron will emit two photons (of the same wavelength) when itdrops to its lowest energy state. These two photons can excite two electrons (one each), and four photonswill be emitted, and so on. This “negative absorption” causes an increase of photons in the radiationfield. If the number of excited electrons exceeds the number of electrons in the ground state, populationinversion is reached, where the rate of stimulated emission exceeds absorption, and the radiation can beamplified. Let us now consider the interaction between an EM wave traveling parallel to a magnetic field,and an incoming beam of electrons spiraling around the magnetic field lines. Initially the phase of electronsin their orbits around the field lines is random, however if the frequency of the EM wave is close to theelectron synchrotron frequency, phase bunching can occur. The bunches of electrons will simultaneously

3A synchrotron maser can operate at radio frequencies consistent with FRBs of both galactic and cosmologicalorigins (Ghisellini, 2017), and can possibly produce the circularly and linearly polarized emission observed in someFRBs (Long and Pe’er, 2018). Curvature masers have been considered as a source of curvature radiation for FRBs,but are found to be non-viable (Locatelli and Ghisellini, 2018).

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jump to the ground state, and emit photons of the same wavelength to start the chain of stimulated emissiondescribed above, amplifying the radiation and thereby forming a maser.

3.2.3. Dicke’s Superradiance

In quantum optics, Dicke’s superradiance (DSR) is emission from a quantum entanglement of atoms ormolecules. When an atom/molecule emits a photon through an transition, it will do so over some definedtimescale. When a large collection of atoms/molecules are entangled, a transition in one induces transitionsin the others. The atoms will then simultaneously emit photons coherently, at a higher energy over a shortertimescale than if they were not entangled. There is also a characteristic delay of DSR associated with bulkemission.

4. Progenitor Theories

We now dive into our main agenda for this article: the cataloguing of various progenitor theories and theircounterparts for experimental verification or exclusion.

4.1. Compact Object Mergers / Interactions

4.1.1. Neutron Star–Neutron Star Mergers / Interactions

In this section we consider three theories of a similar kin and then give the counterparts expected in suchscenarios.

4.1.1.1. Magnetic Braking. Let us consider the merging of two differentially rotating neutron stars (NSs)into a uniformly rotating hypermassive NS. Upon coalescence, the merger spins down and magnetic brakinggenerates coherent radiation. Since the merger rate of NSs is consistent with the expected FRB rate (Totani,2013), this model implies that a large fraction of NS mergers must produce FRBs. Significant mass ejectionsare likely to occur during the merger process, which render FRBs unable to penetrate the ejecta (Hotokezakaet al., 2013). There is a ∼ 1 ms time frame after the maximum rotation speed of the merger is reached andbefore the release of dynamical ejecta, during which the transmission of a single FRB is possible. Once theejecta has sufficiently dissipated ∼ 1− 10 years after the merger, pulsar wind nebula energized by the NScould be a source of repeating FRBs (Yamasaki et al., 2018) (see Section 4.3.3 for further detail.)

4.1.1.2. Magnetic Reconnection. A unipolar induction model has been used to study NSs during the finalstages of their inspiral (Wang et al., 2016). As the stars approach each other, a toroidal magnetic field isinduced around the star’s individual magnetic field lines. The toroidal field strength eventually builds upto rival the poloidal field of the magnetospheres, resulting in magnetic reconnection. After reconnection,the toroidal magnetic field becomes weak again and the process can repeat as the NSs continue to spiralinwards. If emission occurs for two orbital periods, a double peaked FRB could be observed, as in FRB120012 (Champion et al., 2016). In this theory, the merger product is presumed to be a rapidly spinningblack hole (BH).

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4.1.1.3. Changing Magnetic Flux. A close encounter between two NSs results in the formation of highlyelongated orbits, continuously shrinking due to dissipation through gravitational waves (GWs). As theorbital separation decreases, the changing magnetic flux bunches particles, which then radiate coherently.The NSs will approach each other several times, during which their respective magnetospheres will interact,leading to several FRBs before the final merger (Dokuchaev and Eroshenko, 2017). Optimistic predictionsindicate that the expected collision rates in galaxy centres are consistent with FRB observations.

Expected counterparts to NS mergers are GWs (Abbott et al., 2017), X-rays (Rees, 1976; Brown et al., 1996;Hansen and Lyutikov, 2001; McWilliams and Levin, 2011; Lai, 2012), sGRBs and afterglows (Paczynski,1986; Eichler et al., 1989; Narayan et al., 1992; Dai et al., 2006; Zhang, 2013), and kilonovae (Li andPaczynski, 1998; Metzger et al., 2010). The sGRBs themselves are not necessarily observable; the directionof curvature radiation doesn’t always align with the orbital angular momentum along which sGRBs jetsare emitted. The short duration GRB afterglow, with its wider opening angle, would be a better signatureto search for. In a kilonovae, as the stars coalesce, radioactive elements formed in a rapid neutron captureprocess (r-process) decay to power optical or near infra-red (NIR) emission. The latter is likely insignificant(Peng et al., 2018), but optical emission may be observable. The signal is isotropic and detectable for aperiod lasting days to weeks, making kilonovae good candidates for observational detection of counterparts.It is difficult to distinguish kilonovae from supernovae, but the rapid decay and/or color evolution ofkilonovae could help differentiate them (Niino et al., 2014). Note that GW 170817 has been observed witha kilonova and EM counterparts, however follow up searches for FRBs have yielded null results (Andreoni,2017).

4.1.2. Neutron Star–Supernova Interactions

One of the first theories posited that a millisecond pulse, akin to the Lorimer burst, could be formed whena supernova shock interacts with the magnetosphere of a NS in a giant binary (Egorov and Postnov, 2009).When the shock encounters the NS magnetosphere, it sweeps out a magnetospheric tail, which triggersreconnection and hence emission. A GRB is expected in such an event, but with a low flux that may bedifficult to detect.

4.1.3. Neutron Star–White Dwarf Mergers

Next we consider the interaction between the bipolar magnetic fields of a NS and a magnetic white dwarf(WD) as a possible origin of repeating FRBs (Gu et al., 2016). As the WD exceeds its Roche lobe, theNS accretes the infalling matter. Upon their approach, the magnetized materials may trigger magneticreconnection and emit curvature radiation. In a rapidly rotating NS, the angular momentum added byaccretion is lost to gravitational radiation, but the mass transfer may be violent enough for the angularmomentum of the WD to dominate over the gravitational radiation. In this case, theWD is kicked away fromthe NS, and the process of accretion, and thus magnetic reconnection, may repeat. The timescale of emissionis assumed to be the same as that of magnetic reconnection, and the time interval between adjacent burstsis derived from its relationship to the mass transferred by the first burst. There are multiple parametersets that can describe a repeating FRB, an example of which produces timescales roughly consistent withFRB 121102. Counterparts to this model are not specified, other than to say that possible γ-ray emissionfrom synchrotron radiation is unlikely detectable.

The emission of a single FRB upon NS-WD coalescence has also been suggested (Liu, 2018). As they merge,magnetic reconnection injects relativistic electrons from the surface of the WD into the magnetosphere ofthe NS to create a burst of coherent curvature emission. The timescale of the burst is assumed to be fromthe time of electron injection to the formation of the final merged object. It is predicted that the shorterthe intrinsic width of the FRB, the higher the flux density. Of the 28 FRBs analyzed (those available atthe time), the pulse widths were broader than expected in the NS–WD scenario, but perhaps pulse widths

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vary more widely between FRBs due to multipath scattering through the IGM (Cordes and Lazio, 2002;Ravi, 2017). As more FRBs are detected and further constraints are placed on pulse widths, this theorycan be tested.

4.1.4. Binary White Dwarf Merger

It has been proposed that FRBs may form when a doubly-degenerate binary WD merger forms a rapidlyrotating, magnetized, massive WD (Kashiyama et al., 2013). The event rate of such a scenario is consistentwith that predicted for FRBs. The rapid rotation of the WD merger transports, via convection, innermagnetic fields to the polar regions, which greatly enhances the magnetic field strength. The correspondingenergy budget has been shown to be sufficient for FRB production. In the polar regions, where themagnetic fields are twisted by differential rotation or magnetic instabilities, reconnection is triggered, andelectron bunches are injected into the polar region in a timescale comparable with FRBs. The electronsare accelerated to relativistic speeds along magnetic field lines, creating curvature radiation. WDs transferangular momentum into the surrounding debris disk, rapidly reducing their rotation speed and hamperingmultiple FRB events. The FRBs are predicted to have one of two possible counter parts: X-ray emission,or a Type Ia supernova (SN) formed upon the collapse of the massive WD. Whether FRBs can penetrateSN ejecta has, however, been called into question (see Section 4.3.3).

4.1.5. White Dwarf–Black Hole Mergers

During the merger of a BH and a WD, a transient accretion disk is expected to form around the BH, whichcould power a high speed wind around the entire BH-accretion disk system, forming a corona (Dong et al.,2018). Closed magnetic field lines, emerging continuously between the accretion disk and the corona, aretwisted by the turbulence in the system, leading to the formation of rope-like flux structures in the corona(Yuan et al., 2009). When the threshold for mass equilibrium is exceeded, the rope is thrust outward asan episodic jet of relativistic magnetized plasma; a so-called “magnetic blob”. Before the accretion disk isexhausted, 2–3 magnetic blobs could be ejected at different speeds and will collide at a time after ejection(Li et al., 2018a). The collision causes catastrophic magnetic reconnection, and the release of magneticenergy is propagated through the magnetized cold plasma of the blob, and converted to particle kineticenergy. The resulting synchrotron maser could power a non-repeating FRB (Li et al., 2018a). The expectedduration, frequency and energetics in this scenario are consistent with FRBs, and the event rate of BH–WDmergers is compatible with that expected for non-repeating FRBs (Li et al., 2018a). Note that the accretiondisk is advection-dominated. If the disk has a neutron-dominated accretion flow, only a single blob can beejected within the lifetime of the accretion disk, and thus no collision will take place. X-ray emission fromthe accretion disk is expected, which will last only as long as the transient disk itself (Li et al., 2018a).

4.1.6. Neutron Star–Black Hole Mergers

During the inspiral of a NS–BH merger, the magnetic field lines of the NS may thread around the BHevent horizon in a way similar to a battery powering a circuit (McWilliams and Levin, 2011)—the NSacts as an external resistor, the magnetic field lines as wires, the magnetospheric e−e+ plasma providingcurrent, and the BH provides the power. The charged particles in the NS magnetosphere are propelled alongmagnetic field lines by the BH (Ardavan et al., 2009) and generate radiation akin to FRBs (Mingarelli et al.,2015). Under usual circumstances the coalescence of such a system is expected to remain dark; most non-spinning BHs will engulf a companion NS whole, with the disruption radius within the Schwarzchild radius,preventing EM radiation from escaping. In this scenario, however, the battery phase generates high energyX- and γ-rays. The FRB emission would have a distinct signature, with a precursor and a double peak. Theprecursor is a rapid increase in luminosity, 20–80% as bright as the main burst. The double peak consists

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of a main burst prior to the merger and a post-merger burst, where the former is the peak luminosity, andthe latter is from a shock created when the NS’s magnetic field migrates to the BH. While the duration andluminosity of NS–BH radio bursts have been shown to be consistent with FRB observations, the estimatedFRB event rate used at the time was approximately three orders of magnitude greater than the optimisticestimate of the NS–BH merger rate (Mingarelli et al., 2015). A more recent estimate, however, reduces theFRB event rate by an order of magnitude (Champion et al., 2016; Lawrence et al., 2017), thereby decreasingthe discrepancy.

4.1.7. Pulsar–Black Hole Interactions

It has been proposed that an enhanced giant pulse generated by a rapidly spun-up NS near a spinningBH of mass M ∼ 12M⊙ could produce a non-repeating FRB (Bhattacharyya, 2017). A gyroscope is usedto model the pulsar’s spin-precession, which has been shown to increase rapidly near the event horizon ofa Kerr black hole (KBH) (Chakraborty et al., 2017). Eventually the spin precession exceeds the pulsar’sspin, and the latter can be neglected. As such, the pulsar magnetosphere is essentially rotating aroundthe spin-precession axis. The rapid spin-up causes a giant pulse (see Section 4.3.1), whose emission isconsistent with an FRB. The event rate of FRBs is also consistent with predictions of this theory. Thepulsars considered in this scenario are fairly old (to account for the time taken for the NS and BH to merge)and therefore SN not are not expected. The presence of EM counterparts are uncertain, but GWs may bedetectable. If two or more bursts are released during the rapid spin-up, the event duration is expected to be> 1ms for an unresolved burst and a double peaked profile is expected if the burst is partially resolved. Thecalculations used invoke a point gyroscope, which could affect the results significantly. Also, the influenceof the spin-precession and the tidal force of the BH on the pulsar and its magnetosphere has not beenconsidered. Further investigation into the theory is thus required.

4.1.8. Kerr-Newman–Black Hole Interactions

Next we investigate the mergers of Kerr-Newman black hole (KNBH) systems—spinning BHs that carry aelectric charge.

4.1.8.1. Inspiral. A binary BH system in which at least one of the spinning BHs carries a charge wouldinduce a global magnetic dipole normal to the orbital plane. During inspiral, as the orbital separationdecreases, the magnetic flux of the system changes rapidly, which leads to particle bunching and the emissionof coherent curvature radiation. And for some minimal values of the charge of the BH, this scenario couldproduce an FRB and a sGRB (Zhang, 2016a). The detection of both signals could provide a lower limit onthe charge, and the non-detection of a sGRB could provide an upper limit. Lending support to the modelitself, the association of a sGRB with GW 150914 (Connaughton et al., 2016) can be consistently explainedin this scenario (Zhang, 2016a). However, the estimated merger rate of binary BH systems is ∼ 20 timessmaller than that estimated for FRBs, and thus the theory can only account for the full population in theunlikely case that only . 5% of FRBs are of cosmological origins.

4.1.8.2. Induced Magnetosphere Collapse. Magnetospheric instabilities in KNBHs are studied in (Liu et al.,2016), in which the authors use a test particle to show that if the magnetospheres of KNBHs have unstableregions, with a certain charge of the particle, and charge and spin of the BH, the resulting emission is con-sistent with FRB observations. For merging BH binaries in which at least one of the BHs is Kerr-Newman,instabilities due to tidal forces induce reconnection in the KNBH prior to coalescence. The magnetic fieldviolently reconnects, triggering strong relativistic shock waves through the surrounding plasma to producecurvature radiation. γ-rays could occur when relativistic particles associated with the shock are cooledthrough synchrotron radiation and inverse Compton scattering (Gao et al., 2013). The resulting FRB

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would be a precursor to a GW event. To differentiate these events from NS mergers one must note: KNBHshave no thermal emission because they lack solid surfaces, and are non-pulsating because their magneticand rotation axes are aligned. In order to observe the FRBs, the BH spin must be pointing towards theobserver. As with the previous BH–BH merger scenario, this model can only account for a sub-populationof FRBs.

4.2. Collapse of Compact Objects

4.2.1. Supramassive Neutron Star to Kerr-Newman Black Hole

Upon the collapse of a supramassive NS into a BH, an event horizon will likely form before most of the massand radiation can escape. Here we specifically consider an isolated and magnetized supramassive rotatingNS that collapses into a Kerr-Newman BH. By the no-hair theorem, magnetic fields are forbidden frompiercing the event horizon, and so the magnetosphere will be left behind. Although applicability of theno-hair theorem to NS collapse has been questioned (Lyutikov and McKinney, 2011), numerical simulationssuggests it holds (Dionysopoulou et al., 2013; Lehner et al., 2012; Nathanail et al., 2017). Further, withinthe same constructs of this FRB model, the no-hair theorem can be avoided (Punsly and Bini, 2016): if a NScollapses into a metastable KNBH, its electric discharge can cause the magnetosphere to be shed. Violentmagnetic reconnection outside the horizon would then induce a strong magnetic shock wave that movesthrough the remaining plasma at the speed of light, resulting in curvature emission could produce a non-repeating FRB (Falcke and Rezzolla, 2014). The timescales and energetics are consistent with FRBs (Mostet al., 2018). The relatively clean environment is unsuitable for generating GRBs, and thus gravitationalwaves are the only expected FRB counterparts. If, however, the supramassive NS collapses into a BH shortlyafter its birth, an X-ray afterglow and short/long GRB may be observable prior to the FRB (Zhang, 2014).In order to attain the required event rate, ∼ 3% of NSs must be supramassive, rapidly rotating, and highlymagnetized. Whether enough of these NSs can form is uncertain and such formation mechanisms are largelyunknown.

4.2.2. Neutron Star to Quark Star

As a NS spins down, the reduction in centrifugal forces cause the density in the core to increase to the pointwhere neutrons may split in to their constituent parts—a process known as quark deconfinement. Thisphase transition from neutrons to a quark-gluon plasma triggers a massive explosion—a quark nova—inwhich the parent NS collapses into a quark star (Itoh, 1970; Ouyed et al., 2002; Staff et al., 2006; Ouyedet al., 2013). The outer layers of the NS are ejected at relativistic speeds, generating highly unstable rapidneutron-capture (r-process) elements, which undergo a rapid series of β decays (Jaikumar et al., 2007;Charignon et al., 2011). The electrons emitted from this decay stream into the magnetosphere to generatesynchrotron emission. Although the duration of the burst is predicted to be over several seconds, as opposedto the millisecond timescale observed for FRBs, it has been argued that the use of a de-dispersion processthat stacks frequency channels to a common initial time is incorrect, and that the observed pulse is in facta series of self-similar signals at different frequencies that takes place on a larger timescale (Shand et al.,2016). If this is the case, the observed DM would be far lower in actuality. The expected counterparts areGWs from the explosion and from the quark star oscillation modes (Gondek-Rosinska et al., 2003; Floresand Lugones, 2014), and X- and γ-ray emission directly from the quark star (Ouyed et al., 2011).

4.2.3. Dark Matter Induced Neutron Star Collapse

It is possible that a NS could capture ambient dark matter particles as they scatter off the NS nucleonsand become gravitationally bound. Once the dark matter particles thermalize to the NS temperature, they

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sink to the center of the NS. Here they accumulate until they reach a critical mass and collapse into a BH(Bramante and Linden, 2014). The BH will then engulf the NS, ejecting the NS magnetosphere, causingviolent magnetic reconnection. The resultant coherent curvature radiation may be consistent with a singleFRB (Fuller and Ott, 2015). The lifetime of a NS undergoing dark matter-induced collapse is proportionalto the density of dark matter in its local environment. In regions of low dark matter density, NS lifetimesare of the Hubble scale, however where the dark matter densities are high, the final NS collapse may beobservable today. Origins are thus expected to be central regions of high density galaxies—i.e. massivespirals, early type galaxies, and central cluster galaxies—and the event rate is consistent with that of FRBs(Champion et al., 2016; Lawrence et al., 2017). No transient counterparts are predicted, and (because ofdark matter annihilation) no galactic center X-ray or γ-ray excess (Fuller and Ott, 2015). Whether ornot the properties of dark matter are capable of inducing NS collapse remains speculative (Bramante andLinden, 2014). If NSs in central galaxy regions are found to be older than the expected dark matter-inducedcollapse time, the rate of dark matter accretion is likely too low to form BHs. Further, if one does not seea large population of BHs with masses akin to NSs, the theory must be ruled out.

4.2.4. Collapse of Strange Star Crust

Charge separation in strange quark stars (SSs) can induce large electric fields emanating from their core,which, through polarization of nearby surrounding hadrons, can lead to the formation of a hadronic crustaround the star (Alcock et al., 1986). Should the SS accrete a sufficient amount of matter, the hadrons inthe crust will tunnel across the Coulomb barrier, to the strange quark matter (SQM) core, where they tooare converted to SQM (Zhang et al., 2018). This accretion heats the core, hastens the tunneling process,and eventually and inevitably leads to the collapse of the hadronic crust (Kettner et al., 1995; Huang andLu, 1997). As it collapses, the magnetic field lines associated with the crust are dragged with the matter,causing a disruption in the field lines of the SS core. Thus, via magnetic reconnection, e−e+ pairs areaccelerated to ultra-relativistic speeds along the magnetic field lines, generating a thin shell of relativisticparticles that accelerate around the bare SQM core to emit curvature radiation (Zhang et al., 2018). Even asmall portion of the magnetic energy held in the polar cap region of the SS core would be sufficient to poweran FRB and the timescales of collapse are consistent with observations. There is little thermal radiation inthis scenario, and thus no counterparts at X- or γ-ray frequencies are likely to be observed.

4.3. Supernovae Remnants

In FRB theories, pulsars and magnetars immersed in SNe have been a popular line of investigation; theformer invokes high spin-down energies and the latter strong magnetic fields. The rich ejecta of youngmagnetars or pulsars can provide the observed DM and their expanding ejecta could in principle producethe required high RMs (Michilli et al., 2018). Appropriate NS candidates may occur in neighboring galacticcenters (Pen and Connor, 2015) and—as is consistent with the localization of FRB 120112—star-formingregions of the host galaxy (Kulkarni et al., 2015; Xu and Zhang, 2016).

4.3.1. Giant Pulses

Let us first consider rotationally-powered FRBs from pulsars.

4.3.1.1. Young Rapidly Rotating Pulsars. Analogous to models for the Crab pulsar, FRBs of extragalacticorigins may be giant pulses of a young pulsar (Keane et al., 2012; Cordes andWasserman, 2016; Connor et al.,2016). The mechanism for coherence in pulsar emission is currently an open question, however curvatureradiation by particle bunching is a strong candidate—consistent with FRBs—and has been explored indetail Yang and Zhang (In Press). A specific giant pulse mechanism has been proposed for FRBs, in

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which a nearly charge-neutral clump of particles (produced by a two-streaming instability or a bunchinginstability) is accelerated through the pulsar magnetosphere by some reconnection event. The resultantcoherent curvature radiation will be emitted for the duration that the clump remains intact (Cordes andWasserman, 2016). Note that this scenario represents only one possibility. FRBs are predicted to berepeating and stochastic (Connor et al., 2016). A SN explosion a few years prior to the FRB may beobservable (Lyutikov et al., 2016), however, in contrast to the giant flare model that follows, giant pulsesare not expected to have observable higher energy signals (Bilous et al., 2012; Mickaliger et al., 2012; Aliuet al., 2012; Lyutikov et al., 2016; Lyutikov and Lorimer, 2016). The DM, RM and polarization of FRBsin this scenario are owed to the nebula surrounding the pulsar as opposed to the IGM. This places FRBsat extragalactic (as opposed to cosmological) distances (Connor et al., 2016), and thus relaxes the energyrequirements. In support of the theory, it has been shown that the high RMs can be achieved with apulsar wind nebula (PWN) (Piro and Gaensler, 2018). Repetitions of FRBs would continue as a pulsarspins down. For Galactic pulsars whose ages are older than the spin-down timescale, the radio luminosityeither increases or remains nearly constant as the the spin-down luminosity of the pulsar decreases (Szaryet al., 2014). For the young pulsars considered here, on the other hand, the spin-down luminosity decreaseswithin a timescale of a few years (Kisaka et al., 2017; Lyutikov et al., 2016). The giant pulse model thereforedepends on the observation of rapid flux decay in FRBs within a few years (Kisaka et al., 2017).

4.3.1.2. Schwinger Pairs. In a pulsar that is born with an extremely high spin and a magnetic field com-parable to that of a magnetar, the induced electric field in the magnetosphere may be capable of drawinge−e+ pairs from the magnetosphere vacuum (Lieu, 2017) via the Schwinger mechanism (Schwinger, 1951).These so-called Schwinger pairs are then accelerated in opposite directions along the magnetic field lines toneutralize the electric field, causing oscillations in the field about zero. Schwinger pairs are created in thepolar cap region, where the electric field is strongest, and the coherent curvature emission escapes alongthe the open magnetic field lines. The duration over which the pairs are produced corresponds to that ofan FRB (Lieu, 2017). A repeating FRB is not expected, unless some event causes the NS to spin-up to itsinitial state. Such events are likely quite rare, and are unable to account for the full population of FRBs.

4.3.1.3. Pulsar Wind Bubble. Consider a pulsar (NS or MWD) within a nebula. The dissipation of spinenergy drives a PWN observable as a shell around the NS. The plasma wind ceases at the terminationshock, where the plasma decelerates to sub-relativistic speeds and forms a wind bubble around the pulsar(Dai, 2004). A subsequent outburst, possibly triggered by pulsar spin-down or by magnetic dissipationin the magnetosphere (Thompson and Duncan, 1995), will rapidly decelerate when it impacts the PWN,triggering a GRB. Energy that is not radiated away by the explosion itself travels outwards at a relativisticspeed, causing a highly relativistic shock wave to propagate forward into space (Dai, 2004). Synchrotronemission is generated (Murase et al., 2016), however the coherence mechanism to generate FRBs at thispoint is unknown. A synchrotron maser might result from the coherently reflected particles in the shockfront (Gallant et al., 1992). This coherence mechanism has been considered (Murase et al., 2016)—as it wasoriginally for the FRB-magnetar case (Lyubarsky, 2014) (see next section)—however for NSs and MWDs,the frequency is likely too low to be consistent with FRBs. A reverse shock wave may give rise to afterglow(Yang et al., 2016), however both this afterglow and the GRB formed at the shock front are not expectedto be observable. Emission from the PWN as it expands outwards might be detected in the NS scenario,but not in the MWD scenario (Murase et al., 2016). An SN explosion a few years prior to the FRB may beobserved for either body (Lyutikov et al., 2016). X-rays may be emitted in some pulses, however no FRBshave been detected in regions that the XMM-Newton X-ray telescope covers (Popov and Pshirkov, 2016).

The low efficiency of rotation-powered FRBs has been called to question (Lyutikov, 2017). The problemmight be overcome for SN with a low ejecta mass; as is the case if the SN is ultra-stripped or sufficientlyyoung (Kashiyama and Murase, 2017), however the locations of such SN are inconsistent with the hostgalaxy of FRB 121102 (Metzger et al., 2017). The association of a persistent radio signal to FRB 121102

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presents a larger obstacle: the highest luminosity, to date, observed for a Galatic PWN is only 2 × 10−6

times that of the FRB’s counterpart (Michilli et al., 2018). Further, the spectra of bursts from FRB 121102are well-modeled by a Gaussian distribution (Law et al., 2017; Scholz et al., 2016), whereas radio pulsarbursts follow a power law distribution (Kramer et al., 2003; Jankowski et al., 2018).

4.3.2. Giant Flares in Magnetars

4.3.2.1. Magnetar Wind Bubble (Single Flare). One of the first postulations for the Lorimer burst wasa magnetar hyperflare (Popov and Postnov, 2007). Since then the idea has been widely considered andbuilt upon (Lyubarsky, 2014; Murase et al., 2016; Beloborodov, 2017). An FRB model analogous to thepulsar wind bubble model above has been proposed (Lyubarsky, 2014), with the power now deriving fromthe magnetic energy of a magnetar, and the shock resulting from a giant flare4 impacting the magnetarwind nebula (MWN). A powerful synchrotron maser consistent with FRBs is formed at the terminationshock, either by magnetic reconnection or a ring-like distribution of gyrating particles at the shock front(Lyubarsky, 2014; Beloborodov, 2017). A defining feature of such emission is a hump in the nebula spectrumnear the nebula’s self-absorption frequency (Yang et al., 2016).

4.3.2.2. Magnetar Wind Bubble (Clustered Flares). The shock front of the nebula in the previous scenariocannot recover fast enough to account for the repeating bursts of FRB 121102 and the energy requirementsare excessive (Beloborodov, 2017). As such, the theory has been adapted (Beloborodov, 2017). A hyper-active magnetar is proposed to produce multiple millisecond flares at different energies close to the magnetar.Such a magnetar is young with a hyper-energetic SN shell and an ultra-fast rotation period. The multipleflares interact to form a series of shocks before reaching the MWN. As in the previous section, the FRBsarise from a synchrotron maser formed by gyrating particles at the shock front. Flares in this scenario arisefrom ambipolar diffusion in the magnetar core; a process which is then enhanced by the strong magneticfields associated with the high magnetar spin. The flares will therefore be significantly more energetic thanthose of usual magnetars. Less active magnetars can emit FRBs by the same mechanism, but these will benon-repeating. Since repeating FRBs call for rarer magnetars, their event rate is expected to be lower.

From these models, high-energy GRBs (Yu, 2014; Murase et al., 2016; Lyutikov and Lorimer, 2016; Metzgeret al., 2017; Beloborodov, 2017) and possibly a coincident optical flash from the explosion (Lyutikov andLorimer, 2016) are predicted. Flaring from the reverse shock could lead to additional (lower-energy) γ-rayemission, and the interaction between the GRB and the ejecta could lead to broadband afterglow emissionlasting days to weeks (Kulkarni et al., 2015; Murase et al., 2016; Lyutikov and Lorimer, 2016). The quasi-steady nebular emission of the magnetar wind nebula itself may be difficult to detect (Murase et al., 2016;Reynolds et al., 2017). X-rays are able to penetrate the ejecta, but only on a & 100 year timescale, andare therefore unlikely to be detected (Margalit et al., 2018). A testable signature is that the persistentvariable radio source associated with FRB 121102 is predicted to decay by ∼ 10% within the next few years(Metzger et al., 2017).

A magnetar that emits bursts at irregular intervals is a soft gamma repeater (SGR). Although SGRs andFRBs share similar properties, such as: characteristic timescales, low duty factors and repetition (Popovand Postnov, 2007, 2013; Thornton et al., 2013; Kulkarni et al., 2014; Lyubarsky, 2014; Yu, 2014; Pen andConnor, 2015; Katz, 2016b,c,d; Murase et al., 2016; Wang and Yu, 2017; Beloborodov, 2017; Metzger et al.,2017; Lieu, 2017), there is a crucial difference. SGRs are observed to be entirely thermal with frequenciesabove the X-ray range, whereas FRBs are observed in radio frequencies. Another possible inconsistency isthat the Parkes Telescope failed to detect an FRB counterpart to the giant flare of SGR 1806-20: only one

4One proposed flare candidate is that created in the Schwinger pair model presented in the previous section (Lieu,2017)

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of the fifteen FRBs analyzed has a γ-ray fluence ratio consistent with the SGR (Tendulkar et al., 2016). Thebursts of FRB 121102 have varied spectral characteristics, which suggests the observed fluence ratio mayvary significantly between different magnetars and between bursts from the same magnetar. FRBs thereforemay not be observable for all SGRs, which would explain the lack of a detectable radio counterpart in SGR1806-20 (Tendulkar et al., 2016). Searches for GRBs associated with FRBs (repeating and non-repeating)with the Fermi Large Area Telescope (Fermi LAT) have not revealed any results, nor have placed anystringent constraints on the magnetar model (Xi et al., 2017; Zhang and Zhang, 2017). For details on theoptimal observing windows for follow up observations of FRBs associated with SGRs, see Ravi and Lasky(2014).

The flares from young magnetars are consistent with the properties of the Lorimer burst (Popov andPostnov, 2007) and with the Thornton et al. (2013) FRBs (Popov and Postnov, 2013). FRB 110523 isin-keeping with magnetar flares, too (Keane et al., 2016). Based on the observations of SGR 1806-20, theenergy and number of particles (N ∼ 1052) in FRB 121102 are found to be consistent with magnetar ejecta,and thus it is arguably more likely to be powered by magnetic fields than rotational energy (Beloborodov,2017). The host galaxy of FRB 121102 supports the predicted LGRB and hydrogen-poor superluminoussupernovae (SLSNe) formed in the birth of millisecond magnetars (Metzger et al., 2017; Nicholl et al., 2017).The variable radio source associated with FRB 121102 is consistent with the giant flare theory, too: it maybe emission directly from the MWN, the shock interaction between the flare and the MWN, or afterglowfrom an off-axis long gamma ray burst (LGRB) (such that only the afterglow is observed) (Metzger andPiro, 2014; Metzger et al., 2017). Note that for the flare model to be consistent, this emission is expected todecay by ∼ 10% within the next few years (Metzger et al., 2017). Constraints on the large, decreasing RMand required radio transparency for FRB 121102 is consistent with a young & 30− 100 year old magnetarwith an expanding magnetized electron-ion nebula, akin to those associated with SLSNe (Margalit et al.,2018). Such a nebula can also account for the observed properties of the variable counterpart associatedwith FRB 121102 (Margalit et al., 2018).

The flare theory has been met with various criticisms. The upper limit on RMs of giant flares withrelativistic outflows (Gaensler et al., 2005) is 4 orders of magnitude lower than that observed for FRB121102 (Michilli et al., 2018). The magnetar wind therefore may not have a large enough magnetic fieldto account for the RM without a massive BH in its vicinity (Michilli et al., 2018; Zhang et al., 2018).Should this be the case, the chances of having a young magnetar (∼ 30 − 100 years) near a massive BHmay be lower than having a regular magnetar (Zhang et al., 2018). The polarization percentage of pulsaremission has been observed to generally increase with decreasing observing frequency (Morris D., 1970;Manchester, 1971; Manchester R. N., 1973; Xilouris K. M., 1996; Wang et al., 2015), with some pulsarsalso having a constant linear polarization percentage below some critical frequency (Manchester R. N.,1973). The variation of RM is also expected to increase as the distance to the pulsar increases (Mitra et al.,2003). To be consistent with ∼ 100% linear polarization and varying RM (∼ 10% over 7 months) of FRB121102, FRBs must originate some distance from the surface of magnetar. This presents a conflict (Zhang,2018): the enormous brightness temperatures of FRB emission requires strong magnetic fields close to themagnetar surface (Kumar et al., 2017; Beloborodov, 2017). The age of the magnetar must also fall withina “Goldilocks Zone”: for the appropriate energy budget, the magnetar cannot be too old, but to penetrateejecta and avoid DM variation it cannot be too young (Piro, 2016; Cao et al., 2017; Kashiyama and Murase,2017; Zhang and Zhang, 2017; Yang and Zhang, 2017).

4.3.3. Ejecta Penetration

Many authors have shown that SN ejecta would be opaque to FRB-like signals and could take up to 100sof years dissipate sufficiently for GHz radio waves to penetrate (Piro, 2016; Murase et al., 2016, 2017; Piroand Burke-Spolaor, 2017; Lieu, 2017). SN 1986J has been used as an example to show the ejecta wouldbecome optically thin ∼ 60 − 120 years after the core collapse of the parent body (Bietenholz and Bartel,

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2017). By this time, SN ejecta would be unable to produce a sufficiently high DM. This is consistent withfindings that FRBs must pass through the intergalactic medium as well as the SN ejecta (Katz, 2016c),which strongly implies cosmological origins. It has been postulated that as SN ejecta expands and becomesmore diffuse, the DM of FRB pulses from the same source will vary (Piro and Burke-Spolaor, 2017). Morecomplex calculations in a follow-up paper, however, show that the DM can be constant or even increasing(Piro and Gaensler, 2018). This is consistent with repeating FRB 121102, whose DM appears constant(Michilli et al., 2018). Further searches may show a non-constant DM (Yang and Zhang, 2017), but theSN-FRB theory may remain viable either way. To avoid the ejecta caveat altogether, one may considerthat a pulsar is kicked away from its birth state and that the pulsar wind is generated by dense ISM (Daiet al., 2017).

4.4. Active Galactic Nuclei

Various FRB origins involving AGNs have been suggested: the interaction of an AGN jet with cavitons,the interaction between an AGN and either a KBH or a SS and a scaled down version of an AGN jet. Herewe summarize these models.

4.4.1. AGN Jet Interacting with Cavitons

Let us first discuss the formation of AGN jets. Consider a hot accretion disk formed as matter is capturedand spirals into a moderately sized BH. Some of the in-falling gas and dust is confined to the poles andejected in two relativistic jets (Blandford and Znajek, 1977). Hot gas clouds of varying densities surroundthe BH, forming a toroid that extends a few parsecs from the BH. As the AGN jet interacts with theclouds, it becomes narrowly collimated. The relativistic e−e+-beam encounters material at the center ofthe host galaxy, and strong turbulence is produced by plasma instabilities. The total pressure and theponderomotive force (experienced by a charged particle in an oscillating electric field) cause electrons andions to separate. These regions, called cavitons, are filled by a strong electrostatic field. Electrons from thebeam that pass through the caviton are coherently scattered and emit strongly beamed Bremsstrahlungradiation in pulses, consistent with FRBs (Romero et al., 2016; Vieyro et al., 2017). FRBs may be single orrepeating, with the latter shown to be consistent with FRB 121102 (Vieyro et al., 2017). Radiation mightbe linearly polarized (as observed in FRB 121102 (Michilli et al., 2018; Gajjar et al., 2018), FRB 110523(Masui et al., 2015), and FRB 150807 (Ravi et al., 2016)) if there is a local magnetic field, however the100% polarization degree of FRB 121102 would be difficult to account for in this scenario (Zhang, 2018).The persistent scintillating radio emission from the AGN is an expected counterpart, which agrees withobservations of FRB 121102.

4.4.2. Kerr Black Hole Interacting with AGN

A repeating FRB could be produced when a KBH surrounded by highly magnetized plasma interacts with anAGN (Das Gupta and Saini, 2017). Episodic winds from the AGN may prompt the KBH to intermittentlyaccrete matter. Through the Blandford-Znajek mechanism, in which a magnetic field extracts spin energyfrom the KBH, an unsteady bipolar jet is triggered. As the jet travels through the surrounding plasma,shocked shells could create a synchrotron maser consistent with FRB observations (Waxman, 2017). TheKBH under consideration may form via a series of events: the collapse of a Wolf-Rayet star (a relatively smallhelium star) into a supramassive magnetar that accretes matter until it collapses into a KBH. Precursors tothe FRB may therefore be the SN Type Ib/c explosion in which the magnetar was born, and the resultantneutrinos and GWs. The release of a prompt GRB may also be observable as the magnetar spins down, butonly if the axis of the magnetar is aligned with earth. GWs will also occur when the magnetar implodes toa quark star.

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4.4.3. Strange Star Interacting with AGN

A SS is made up of approximately the same number of up, down, and strange quarks (Itoh, 1970; Freedmanand McLerran, 1978), with a small number of electrons distributed across the star’s surface. Should an AGNwind interact with a SS, it can induce torsional oscillation of the electron layer relative to the positivelycharged SS, which can emit high luminosity GHz radio waves (Mannarelli, 2014), consistent with FRBs(Das Gupta and Saini, 2017). The sporadic nature of AGN wind would induce a repeating FRB. PersistentGWs are expected from the SS due to its r-mode instability (Andersson et al., 2002). If the SS is the resultof a spinning down magnetar, neutrinos and a GW could be released when the magnetar collapses, howeverthis emission need not be close in time to the interaction of the SS with the AGN, making it difficult todraw any associations.

4.4.4. AGN-like Wandering Beams

FRBs may be formed by a scaled down version of an AGN (Katz, 2017b). The jet formation and beamingmechanism is as in the AGN scenario, but the BH under consideration has a mass lower than the SMBHsof AGNs. If the moderately sized BH is set in a turbulent medium, such as a giant molecular cloud in astarburst galaxy, the angular momentum axis of the BH may be large, and the narrowly collimated beamswill randomly change directions. When a beam sweeps across an observers line of sight, it may be observableas an FRB. There will be a persistent variable radio signal as in an AGN, and very soft X-ray/extreme UVemission from the accretion disk of the BH. The latter would be strongly absorbed in the Galactic plane,and thus only observable for FRBs at high Galactic latitudes.

4.5. Collisions and Close Encounters

4.5.1. Neutron Stars and Small Bodies

Next we investigate a class of progenitor theories based on interactions between a NS and other gravita-tionally bound objects.

4.5.1.1. Comet/Asteroid Captured by a NS. A small body, such as a comet or asteroid, captured by thegravitational potential of a NS will free-fall toward it, becoming radially elongated until it exceeds its Rochelimit and breaks apart (Colgate and Petschek, 1981; Katz et al., 1994). The fragments are compressed bythe gravitational acceleration and the magnetic field of the NS, resulting in leading and lagging portionswith the same velocity. In order to nullify the effects evaporation and ionization may have, the body musthave a sufficiently large mass and shear, and is thus predicted to be of Fe-Ni composition (Cordes andShannon, 2008). Infalling matter would be confined to the poles of the NS by strong magnetic stresses,creating an accretion column (Geng and Huang, 2015). If the accretion column travels through a regionwhere electrostatic equilibrium has been disturbed, particles are accelerated to yield γ-ray emission. Whenthe matter impacts the NS, an expanding plasmoid fireball will be launched along the magnetic field lines.Magnetic reconnection at the collision site accelerates e−e+ pairs within the plasma-fan to ultra-relativisticspeeds. The resulting coherent curvature emission is consistent with a non-repeating FRB (Geng andHuang, 2015; Huang and Geng, 2016). The event rate associated with such a theory has been shown to beconsistent with the other predictions (Thornton et al., 2013) and, notably, the impact timescale betweenthe leading and tailing fragments is roughly consistent with the brevity of FRB signals. The model predictsX-ray emission and γ-ray emission from inverse Compton scattering, however these are probably too faintto be observed (Geng and Huang, 2015).

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4.5.1.2. Pulsar Travelling Though Asteroid Belt. Consider an asteroid belt surrounding a star. If a pulsarpasses through this system, it is likely to encounter multiple asteroids. When this happens, charged particlesmay be stripped from the asteroidal surface into the NS magnetosphere, where they are accelerated to ultra-relativistic speeds. The resulting coherent curvature radiation is consistent with FRB properties (Dai et al.,2016). Further, the time between edge on collisions within the asteroid belt is consistent with the timebetween the signals of FRB 121102. If the pulsar is in a binary system with the star that hosts the asteroidbelt, it could pass through the belt multiple times (Dai et al., 2016; Bagchi, 2017), creating a series ofbursts twice during each orbital period (Bagchi, 2017).

4.5.1.3. Body Orbiting Pulsar. If an orbiting body is massive enough to survive a close encounter withoutevaporation or breaking up (such as a planet or WD) (Kotera et al., 2016), the highly magnetized pulsarwind will induce an EM field around the body. In this situation, Alfven wings are created as the pulsar windcombs the field lines from the nearest pole of the orbiting body and into space. The Alfven wings destabilizethe plasma near the body’s surface to excite coherent emission. Far from the pulsar companion, the emissionis convected with the wind traveling relativistically along the Alfven wings to form a synchrotron maser,whose emission is consistent with FRBs (Mottez and Zarka, 2014). The emission is only observable whenthe companion is aligned between the pulsar and Earth, and thus should repeat periodically—a feature yetto be observed for FRBs. The signal would be composed of one to four peaks, a few milliseconds each, withan event duration less than a few seconds. No emission counterparts are expected, as synchrotron emissionfrom a hot plasma component would be incoherent and thus too weak.

4.5.2. Collisions Between Neutron Stars and Primordial Black Holes

FRBs may result from interactions between NSs and primordial black holes (PBHs) (Abramowicz et al.,2017). As a PBH passes through a NS, the gravitational drag from the dense NS matter causes the PBH toslow down. The PBH will pass through the middle of the NS and, after losing sufficient kinetic energy, willbe pulled back. The PBH will oscillate a few times before settling at the center of the NS. Here, the PBHwill begin to accrete the NS until it is swallowed, causing the NS magnetosphere to be shed. The resultingmagnetic reconnection releases an FRB. A repeating FRB may also be accounted for in this scenario: asmall PBH will take longer to accrete the NS; as the NS is gradually consumed, multiple bundles of magneticfield lines within the NS may be reconfigured, causing multiple bursts. GWs are expected counterparts,but may not be detectable at cosmological distances. The model can account multiple peaks, polarizedemission and Faraday rotation.

4.5.3. Interactions Between Axions and Compact Bodies

Several progenitor theories have been proposed, which involve axion clusters or clouds coming into closecontact with a highly magnetized object. Axions are a prominent candidate for dark matter, and it isexpected that they would evolve into clumps of stellar mass, known as axion clouds and stars. At this stageit is worth noting that axion star-FRB models rely on the assumption that axions form a large fraction ofthe total dark matter component (Eby et al., 2015).

4.5.3.1. Axion Star and Neutron Star. In the presence of a magnetic field, axions have been shown toproduce radiation by generating an oscillating electric field, causing nearby electrons to radiate coherently.The radiation produced when an axion star collides with a NS has been shown to be consistent with non-repeating FRBs (Iwazaki, 2014, 2015a,b). As the axion star moves through the magnetosphere of the NS,a time-dependent electric dipole moment is induced, forcing free electrons above the surface of the NS tooscillate harmonically. This generates coherent radiation with a frequency determined by the axion mass(Iwazaki, 2014)—an effect which could be even larger if one considers the electric dipole moment induced

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in the neutrons interior to the NS (Raby, 2016). The theory is shown to be robust to the effects of tidaldisruption (Iwazaki, 2015b), however this has been disputed (Pshirkov, 2017). A defining feature of themodel is that the intrinsic FRB emission frequency is finite, and the observed spectral broadening is due tothermal Doppler effects (Iwazaki, 2017). The emission is also expected to be circularly polarized (Iwazaki,2014). A two-component profile may be observed if the axion star collides with a binary NS system (Iwazaki,2015b). No counterparts are expected.

4.5.3.2. Axion Star and Black Hole. If an axion star were captured by a BH with a strongly magnetizedaccretion disk, the axion star’s orbit will lead it to approach and impact the accretion disk several times atdifferent locations. The electric field induced by the axion star passing through a strong magnetic field willresult in the coherent oscillation of surrounding electrons. In this scenario the frequency of the radiationwill depend on the velocity of the accretion disk at the point of impact. In this way, the variation in centralburst frequencies of FRB 121102 can be explained. The axion star will likely make several impacts beforeevaporating or eventually being absorbed by the BH (Iwazaki, 2017). As in the previous scenario, theemission would be be circularly polarized, and no counterparts are expected.

4.5.3.3. Induced Collapse of Axion Clumps by a Highly Magnetic Compact Object. Axion clumps withmasses below the stellar range, known as Axion Bose Clusters or “miniclusters”, have been considered asFRB progenitors (Tkachev, 2015). In the strong magnetic field of a compact object, an instability may arisein a minicluster, causing it to explosively decay into photons via a synchrotron maser mechanism (Tkachev,1986). The predicted emission timescale, the energetics, luminosities, and event rate are in-keeping withFRB observations.

4.5.3.4. Superradiant Axion Cloud and Spinning Black Hole. Spinning BHs have superradiant instabilities,and thus may be surrounded by a dense superradiant axion cloud. Similarly to how a laser can be generatedby stimulated axion decay in dense axion clusters (Tkachev, 1987; Kephart and Weiler, 1995), a laser can betriggered in superradiant axion clusters. Such is known as a black hole laser powered by axion superradiantinstabilities (BLAST) (Rosa and Kephart, 2018). For a BLAST’s emission to be consistent with FRBobservations, the required mass dictates that the BHs be primordial. However, because PBHs form whenover densities of gas collapse (Garcia-Bellido et al., 1996), they do not have spin, and are unlikely to spinup via accretion (Ali-Hamoud and Kamionkowski, 2017). The merging of two PBHs is thus considered forFRBs, where the required spin and resultant superradiant instabilities can be induced (Rosa and Kephart,2018). Repeating bursts could occur: the BLAST will form a photon plasma that blocks axion decay andthus halts lasing until e−e+ annihilation reduces the plasma density, and the process can restart (Rosa andKephart, 2018). Observational counterparts could be associated with e−e+ annihilation and/or positronium(a bound particle of an e− and e+), though these are not specified. GWs are also expected.

4.5.3.5. Axion Quark Nugget and Neutron Star. In close analogy with the axion quark nugget (AQN)mechanism for generating solar nano flares (Zhitnitsky, 2017), an AQN falling through an opportunelycomplicated region in a NS magnetosphere may be able to produce sufficient magnetic energy to powerFRBs. Shock waves caused by the infalling AQN would trigger magnetic reconnection, and produce a giantflare (van Waerbeke and Zhitnitsky, 2018). The event rate is consistent with observations by (Connoret al., 2016), but the emission timescale (∼ 10 − 100 ms) is larger than what is observed (van Waerbekeand Zhitnitsky, 2018). This discrepancy can be accounted for if the beam moves across the sky, allowingus only a glimpse of the emission. A curvature radiation mechanism is invoked, which predicts a maximumcut-off frequency at infra-red wavelengths; observed counterparts with higher frequencies would invalidatethe AQN-FRB model. Given the random nature of these events, repeating FRBs would be non-periodic. Acorrelation between the total energy and duration of the flare is predicted, however because only a fraction

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of the entire beam would be observed, this relationship would be difficult to verify (van Waerbeke andZhitnitsky, 2018).

4.6. Other Models

4.6.1. Starquake-Induced Repeaters

The starquakes of a pulsar (Anderson and Itoh, 1969; Baym and Ruderman, 1969; Anderson and Itoh, 1975)have been considered as a source of repeating FRBs (Wang et al., 2018). The bursts of FRB 121102 areconsistent with the aftershock sequence of an earthquake, where the burst’s time-decaying rate of seismicityfalls within the typical values of earthquakes. They also show that the burst energy distribution of FRB121102 has a power law form, much like that of the Gutenberg-Richter law of earthquakes. Further, thewaiting time of bursts has a Gaussian distribution; another characteristic feature of earthquakes. Starquakesare poorly understood, limiting the testability of this theory. They may be associated with SGRs (Chengand Young, 1996; Xu et al., 2006), which offers counterparts for which to search.

4.6.2. Variable Stars

Variable stars may be a source of FRBs (Song et al., 2017), where synchrotron emission is generated byan astrophysical undulator. The model assumes the existence of a weak, axial-symmetric magnetic fieldsome distance from a variable star. The emission frequency will vary relative to the observer, due to thechange in opening angles between the observer and direction of emission as the star rotates. This resultsin a DM consistent with FRBs. Multiple peaks are possible in this scenario. For this model to hold, onemust observe a positive frequency sweep ahead of a negative frequency sweep.

4.6.3. Lightning in Pulsars

Akin to sGRBs powered by the release magnetic energy stored in magnetars, FRBs may be powered by therelease of electrostatic energy stored in pulsars (Katz, 2017a). Provided there are regions of magnetosphericplasma with distinct energy, separated by a vacuum gap, the discharge of such energy could manifest as“pulsar lighting”, analogous to the flow of current in the atmosphere when lightning strikes (Melrose andYuen, 2016). This intense, rapidly varying, electric field in the gaps would accelerate electrons and positronsin the magnetosphere, producing coherent curvature radiation observable as an FRB. The large variationof FRB 121102 burst widths, and hence the variation of spectra fluences and frequencies, may be due toscintillation of pulsar lightning.

4.6.4. Wandering Pulsar Beam

The next model assumes the presence of a steady beam of pulsar emission whose direction randomly changes.If this beam sweeps across the line of sight of an observer, it may be observable as an FRB (Katz, 2016a).The duration of an FRB depends on the speed at which the beam moves across the sky, and hence awandering beam mitigates the enormous power and high spin-down requirements of giant pulse and flaremodels (Section 4.3). This scenario can also consistently explain two pairs of possibly distinct radio burstsdetected in FRB 121102 (Scholz et al., 2017; Hardy et al., 2017). Even with the random walk of the beam,with enough observations one might be able to tease out the periodicity of the pulsar. Details about anemission mechanism or possible counterparts are not specified.

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4.6.5. Tiny Electromagnetic Explosions

The collision of two relativistic macroscopic dipoles that form around the time of cosmic electroweaksymmetry-breaking could cause a tiny5 explosion (Thompson, 2017b). To retain the supporting electricfield for cosmological timescales, these field structures must be superconducting, and are thus dubbedlarge superconducting dipoles (LSDs)6 (Thompson, 2017a,b). The expanding relativistic magnetized shellfrom the explosion couples efficiently to a low-frequency, strong, superluminal EM wave in the surroundingplasma, allowing the emission to escape. Three emission mechanisms are possible: the reflection of anambient static magnetic field by the conducting surface of the shell; direct linear conversion of the magneticfield in the shell; and the excitation of an EM wave if the surface of the shell becomes corrugated viathe reconnection of the ejected magnetic field with the ambient magnetic field. The deceleration of themagnetic shell causes a higher frequency radio pulse and the thermal part of the explosion radiates γ-rays,however the latter are not expected to be detectable (Thompson, 2017b). The model accounts for repeatingand non-repeating FRBs and for their observed differences in linear polarizations and RMs (Thompson,2017a,b)—the hydromagnetic drag on LSDs is weak in the ISM, and strong in high-density environments.As such, a slowly accreting SMBH may capture LSDs and group them in gravitationally bound cusps,within which the LSDs collide and create repeating FRBs. Here the high density plasma accounts for thehigh linear polarization and high RM observed in FRB 121102 (Michilli et al., 2018). The opposite is truefor LSDs far from the SMBH; where the observed RM is low and signals are non-repeating, such as in FRB150215 (Ravi et al., 2016), collisions are expected to take place in dark matter halos of galaxies.

4.6.6. White Hole Explosions

Should a collapsing star reach the Planck density to become a Planck star, it will cease to collapse further(Goswami et al., 2006) and will explode outwards (or bounce) to form a white hole (WH) (Hawking, 1974).Due to their age, PBHs or Planck stars are the strongest candidates to form WHs which may be observabletoday (Carr, 1975; Rovelli and Vidotto, 2014), and the energy they release is consistent with FRBs (Barrauet al., 2018). A single FRB is expected, accompanied by an infra-red signal with a wave length on the orderof the exploding star and γ-rays characterized by the material expelled in the explosion (Barrau et al.,2014).

4.6.7. Neutron Star Combing

Cosmic combing is the process in which the field lines of a NS’s magnetosphere are swept out in a streamby a strong plasma. The effect is caused by ram pressure: the bulk resistance of a fluid acting on an object.When this pressure is greater than the magnetic field pressure, the drag will comb the magnetic field in adifferent direction, causing reconnection with emission consistent with an FRB (Yang and Zhang, In Press;Zhang, 2017, 2018). Combing may occur in a variety of situations, such as: a GRB, a SN, an AGN flare,or a stellar flare. As such, it is a difficult theory to test in general, however a specific scenario has beenconsidered: the combing of a pulsar by an accreting SMBH (Zhang, 2018). An FRB would be observablefor half of the pulsar’s orbital period around the SMBH, implying the signal is periodic (as is suggestedin (Scholz et al., 2016; Price et al., 2018)). This periodicity would not be perfect—the SMBH wind thatinitiates FRBs is variable and thus FRB signals are sporadic. The RM should vary with orbital periodicity,but this would be more difficult to confirm given the sporadic FRB emission. Finally, the polarization angle

5The initial wavelength of the explosion is narrower than the wavelength of the radiation observed so the energytravels outwards in a very thin shell, and the surrounding charged particles are deflected (as opposed to reflected)by the magnetic field embedded in the expanding shell.

6“Large” is of course relative, and is used because the dipoles are macroscopic.

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of each burst within an orbital period would vary depending on the phase of the pulsar’s orbit, and shouldbe correlated with the varying RM.

4.6.8. Neutral Strings

Nambu-Goto (infinitely thin, idealized) strings generically form cusps—portions of the string which foldback onto themselves and move at the speed of light. The cusps decay to form a beam of coherent radiation(Brandenberger, 1987), where the emission can ostensibly be of any energy and frequency range. As such,cusp decay has been considered as an FRB origin (Brandenberger et al., 2017). The event rate, timescale,and flux are shown to be consistent with FRB data, however the relativistic effects on the cusp shape wherenot considered. It has been argued that by taking this into account, the consistency of the theory maybreak down (Costa et al., 2018).

4.6.9. Superconducting Strings

A cosmic string becomes superconducting when coupled with electromagnetism; achievable through theunbroken symmetry of an extra Higgs field in the formation of the string (Witten, 1985). Various mecha-nisms have been considered in which superconducting cosmic strings may produce an FRB, such as: stringoscillations (Vachaspati, 2008), the collisions of string structures (cusps and kinks) (Cai et al., 2012a; Yeet al., 2017), and the interaction of a current-carrying loop in the magnetic field of a galaxy (Yu et al.,2014). In the last scenario listed, the event rate of FRBs indicates a loop size consistent with strings formedduring the radiation era (Yu et al., 2014). The emission from superconducting cosmic strings is linearlypolarized—an intrinsic signature that is independent of frequency and is not affected by polarization viathe ISM (Cai et al., 2012b). Expected counterparts are other EM counterparts—specifically GRBs (Bran-denberger et al., 1993), cosmic rays (MacGibbon and Brandenberger, 1993), and neutrinos (MacGibbonand Brandenberger, 1990)—and GWs.

4.6.10. Dicke’s Superradiance in Galaxies

Postulated in 1953 (Dicke, 1954) and first detected in 1973 (Skribanowitz et al., 1973), DSR has beeninvoked as one of the few “microphysical” FRB models (Houde et al., 2018). The aim is to explain FRBsusing the atomic interactions in galaxies. DSR can occur in astrophysical settings (Rajabi and Houde,2016), provided: the collection of atoms is inverted (to wit, a majority of atoms exist in higher excitedstates than the minority); the velocity coherence is high; and the non-coherent relaxation mechanismsoccur on a timescale larger than the delay time. If one models the ISM as a cylinder of atoms, the predictedDSR emission power and timescale can fit FRB data (Houde et al., 2018). This is because the coherentbehavior of the DSR atoms has a timescale which scales as τ ∝ N−1 and an intensity which scales asI ∝ N2, where N is the number of entangled molecules. The DMs associated with FRBs fits well with theISM required for DSR to occur. The FRBs arising from DSR may be repeating; if a collection of moleculeshas DSR triggered at the same time, the intrinsic variation in the DSR timescale and time delay wouldgive the observation of bursts at different times (Houde et al., 2018). The variation is because the timedelay is an expectation value, and the collection of molecules being ionized at the same time is due tothe entanglement, which also causes a differential in emission time. This process can happen repeatedlyas population inversion will be non-inverted but swiftly restored via the ISM, which will drive more FRBpulses, and so on. The flux distribution of such a setup can be matched to FRB 121102.

4.6.11. Alien Light Sails

Extragalactic, artificial beam-powered light sails have been proposed as an FRB origin (Lingam and Loeb,2017), however such a concept is highly speculative, and cannot be tested nor can it provide predictions.

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4.7. Theories That Have Been Ruled Out

After all the speculation put forth by the community, some models have already been ruled out. Below, forthe sake of completeness, we present a brief outline of the relevant conversations regarding these models.

4.7.1. Stellar Coronae

One potentially promising theory for FRBs of Galactic origins was flare stars. The theory seemed fittingbecause dwarf stars have been observed to produce bursts of coherent radiation on short timescales (< 5 ms)(Lang K. R., 1983; Lang, 1986; Bastian, 1990) and have been observed within FRB fields (Loeb et al., 2014;Maoz et al., 2015). A cyclotron maser in the lower part of the stellar corona could produce a flare consistentwith FRB observations (Loeb et al., 2014), where the large DM and pulse smearing could be attributed tothe corona plasma. Free-free absorption that occurs in the corona, however, presents a problem: a radiosignal from the lower corona with the required DM may be unobservable unless the corona is infeasiblyextended or hot (Luan and Goldreich, 2014). Further, the plasma density required for the DM is arguablytoo high to produce the frequency dependence on the pulse arrival times observed for FRBs. In defenseof the theory, observations by Dennison (2014); Tuntsov (2014) show high flare temperatures capable ofmitigating significant free-free absorption (Maoz et al., 2015). Further, if frequency drifts occur in flares (asobserved in (Osten and Bastian, 2008)), the measured dispersion relationship for FRBs may be possible.Doubt was then cast on the theory again when it was shown that the brightness temperature of FRBs couldnot escape plasma as dense as the DM demands (Lyubarsky and Ostrovska, 2016). The stellar flare theoryfor FRB origins was likely put to rest when constraints by Dennison (2014); Tuntsov (2014); Katz (2016d)were tightened to show the density and expanse of the dispersing plasma required to produce FRBs differsfrom those of stellar coronae by at least an order of magnitude (Masui et al., 2015).

4.7.2. Annihilating Mini Black Holes

When a BH evaporates to some critical mass, a fireball of e−e+ pairs can be created. The relativistic pairsexpand into the magnetic field of the surrounding ISM, which, for a BH of mass MBH < 1013 kg, couldproduce emission consistent with an FRB (Keane et al., 2012). The inferred distance for the Lorimer burstin this scenario, however, is calculated to be . 20 kpc. This would place the source within our galaxy, andthus the theory is rendered void.

5. Conclusion

In this review article, we have catalogued a collection of postulated FRB models and have attempted toprovide the reader with a general overview of the ongoing research in FRB model-building. The theoriesvary in their explanatory power, testability, and sometimes in their “exoticness”. Given both the numberof theories and the range of physics used as a foundational framework, it seems like an ideal time to takestock of the theoretical work produced. The small data set and lack of observational counterparts meansmost of the theoretical work is phenomenological.

5.1. Future Observational Constraints

With a considerable increase in radio data predicted from telescopes such as CHIME, LOFAR, HIRAX,SKA, ASKAP, UTMOST, FAST and Apertif installed in the WSRT, as well as in other frequency bandsby telescopes such as the Fermi LAT, Fermi GBM and Swift/BAT, it is hoped that constraints on manyof the models discussed here will be able to be rule out—and even favor—certain approaches. Here we

27

discuss some of the major ways observations can constrain theory. The constraints are divided into twobroad sections and we discuss each in succession.

5.1.1. Astrophysical Formation Channels

From an observational perspective, the strongest constraints on the astrophysical channel of FRB formationwould likely come from arcsecond-precision localization of FRBs, leading to an understanding of theirenvironments and their progenitor populations. Historically, this mirrors the expansion of our understandingof long and short GRBs after the first afterglow localizations with Beppo-SAX. This would provide the leastcontrovertible evidence of any possible distinction between repeating and non-repeating FRBs—differentburst morphologies or statistical rate calculations can be fraught with biases and complications. Current andupcoming FRB search projects with the ASKAP, HIRAX, UTMOST-2D and CHIME-outriggers promiseto provide a large sample of well-localized FRBs with identifications of their host galaxy environments. Theenvironments of FRBs will also be constrained by observations of rotation measures, temporal variations inrotation and local dispersion measures. Apart from the environments, the host identification will provide arobust distance and energy scaling for the emission mechanism.

5.1.2. Emission Mechanisms

Most theories expect that the radio emission is a small fraction of energy budget. Detecting or constrainingmulti-wavelength or multi-messenger prompt counterparts would place constraints on the emission mecha-nism. The challenge is that the sensitivity of wide-field of view γ-ray, X-ray, and optical/infrared telescopesis much lower (in νLν) compared to that of radio telescopes. The nearest and brightest FRBs will likelylead to the best constraints and are most likely to have detectable prompt counterparts at other wave-lengths. The combination of very wide-field radio telescopes at the GHz frequency band and the all-skyX-ray and γ-ray burst monitors may lead to strong constraints, albeit on a small number of bright FRBs.The polarization and temporal characteristics of FRBs—fraction of linear/circular polarization, change inpolarization angle would also constrain the emission mechanisms of FRBs. Repetition statistics could givephenomenological clues about the energy source and trigger of the emission mechanisms.

Unsurprisingly, we cannot predict the future of FRBs, how the mystery will be solved and how astronomerswill converge on to the answers, but the field certainly will be exciting for many years ahead.

6. Acknowledgements

We would like to thank Renato Costa for many useful discussions during the early part of this work. Wewould like to thank Bryan Gaensler for many useful discussions about radio astronomy, and for introducingthe UCT group to Fast Radio Bursts in the first place. We would like to thank the anonymous refereefor their valuable feedback. E. Platts is supported by a PhD fellowship from the South African NationalInstitute for Theoretical Physics (NITheP). A. Walters acknowledges support from the National ResearchFoundation of South Africa (NRF) [grant numbers 105925, 110984, 109577], and thanks A. Weltman forhosting visits to UCT. S. Tendulkar acknowledges support from a McGill Astrophysics postdoctoral fellow-ship. A. Weltman gratefully acknowledges support from the South African Research Chairs Initiative of theDepartment of Science and Technology and the National Research Foundation of South Africa. J. Gordin,E. Platts and S. Kandhai are all supported by grantholder bursaries under this programme. J. Gordin isfinancially supported by the National Astrophysics & Space Sciences Programme (NASSP). Any opinion,finding and conclusion or recommendation expressed in this material is that of the authors and the NRFdoes not accept any liability in this regard.

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7. Tabulated Summary

A living version of the following tabular summary can be found at http://frbtheorycat.org.

Progenitor Mechanism Emission Counterparts Type References

Merger

NS–NSMag. brak. — GW, sGRB, Single Totani (2013)Mag. recon. Curv. afterglow, X-rays, Both Wang et al. (2016)Mag. flux — kilonovae Both Dokuchaev and Eroshenko (2017)

NS–SN Mag. recon. — None Single Egorov and Postnov (2009)

NS–WDMag. recon. Curv. — Repeat Gu et al. (2016)Mag. recon. Curv. — Single Liu (2018)

WD–WD Mag. recon. Curv. X-rays, SN Single Kashiyama et al. (2013)WD–BH Maser Synch. X-rays Single Li et al. (2018a)NS–BH BH battery — GWs, X-rays, Single Mingarelli et al. (2015)

γ-raysPulsar–BH — — GWs Single Bhattacharyya (2017)KNBH–BH Mag. flux Curv. GWs, sGRB, Single Zhang (2016a)(Inspiral) radio afterglowKNBH–BH Mag. recon. Curv. GW, γ-rays, Single Liu et al. (2016)(Magneto.) afterglow

Collapse

NS to KNBH Mag. recon. Curv. GW, X-ray Single Falcke and Rezzolla (2014)afterglow & GRB Punsly and Bini (2016)

Zhang (2014)NS to SS β-decay Synch. GW, X- & γ-ray Single Shand et al. (2016)NS to BH Mag. recon. Curv. GW Single Fuller and Ott (2015)SS Crust Mag. recon. Curv. GW Single Zhang et al. (2018)

SNR

(Pulsar)

Giant Pulses Various Synch./ — Repeat Keane et al. (2012)Curv. Cordes and Wasserman (2016)

Connor et al. (2016)Schwinger Pairs Schwinger Curv. — Single Lieu (2017)PWN Shock — Synch. SN, PWN, Single Murase et al. (2016)(NS) X-raysPWN Shock — Synch. SN, X-rays Single Murase et al. (2016)(MWD)

SNR

(Mag.)

MWN Shock Maser Synch. GW, sGRB, radio Single Popov and Postnov (2007)(Single) afterglow, high Murase et al. (2016)

energy γ-rays Lyubarsky (2014)MWN Shock Maser Synch. GW, GRB, radio Repeat Beloborodov (2017)(Clustered) afterglow, high

energy γ-rays

AGN

Jet–Caviton e− scatter Bremsst. X-rays, GRB, Repeat Romero et al. (2016)radio Single Vieyro et al. (2017)

AGN–KNBH Maser Synch. SN, GW, γ-rays, Repeat Das Gupta and Saini (2017)neutrinos

AGN–SS e− oscill. — Persistent GWs, Repeat Das Gupta and Saini (2017)GW, thermal rad.,γ-rays, neutrinos

Wandering — Synch. AGN emission, Repeat Katz (2017b)Beam X-ray/UV

29

Collision/In

teraction

NS & Ast./ Mag. recon. Curv. None Single Geng and Huang (2015)Comets Huang and Geng (2016)NS & Ast. e− stripping Curv. γ-rays Repeat Dai et al. (2016)Belt Bagchi (2017)Small Body Maser Synch. None Repeat Mottez and Zarka (2014)& PulsarNS & PBH Mag. recon. — GW Both Abramowicz et al. (2017)Axion Star e− oscill. — None Single Iwazaki (2014, 2015a,b)& NS Raby (2016)Axion Star e− oscill. — None Repeat Iwazaki (2017)& BHAxion Cluster Maser Synch. — Single Tkachev (2015)& NSAxion Cloud Laser Synch. GWs Repeat Rosa and Kephart (2018)& BHAQN & NS Mag. recon. Curv. Below IR Repeat van Waerbeke and Zhitnitsky (2018)

Other

Starquakes Mag. recon. Curv. GRB, X-rays Repeat Wang et al. (2018)Variable Undulator Synch. — Repeat Song et al. (2017)StarsPulsar Electrostatic Curv. — Repeat Katz (2017a)LightningWandering — — — Repeat Katz (2016a)BeamTiny EM Thin shell Curv. Higher freq. Repeat Thompson (2017b,a)Explosions related radio pulse, γ-raysWHs — — IR emission, γ-rays Single Barrau et al. (2014, 2018)NS Combing Mag. recon. — Scenario Both Zhang (2017, 2018)Neutral Cosmic Cusp decay — GW, neutrinos, Single Brandenberger et al. (2017)Strings cosmic rays, GRBsSuperconducting Cusp decay — GW, neutrinos, Single Costa et al. (2018)Cosmic Strings cosmic rays, GRBsGalaxy DSR DSR Synch. — Both Houde et al. (2018)Alien Light Artificial — — Repeat Lingam and Loeb (2017)Sails transmitter

Inviable Stellar Coronae N/A N/A N/A N/A Loeb et al. (2014)

Maoz et al. (2015)Annihilating N/A N/A N/A N/A Keane et al. (2012)Mini BHs

Table 1: Tabulated Summary

30

8. Acronyms

AGN active galactic nuclei. 7, 20, 21, 25

Apertif Apertif Radio Transient System. 27

AQN axion quark nugget. 23

ASKAP Australian Square Kilometre Array Pathfinder. 27, 28

BH black hole. 11, 13, 14, 15, 19, 20, 21, 23, 27

BLAST black hole laser powered by axion superradiant instabilities. 23

CHIME Canadian Hydrogen Intensity Mapping Experiment. 27, 28

CMB cosmic microwave background. 5

DM dispersion measure. 4, 5, 6, 7, 8, 15, 16, 19, 24, 26, 27

DSR Dicke’s superradiance. 9, 11, 26

EM electromagnetic. 9, 10, 11, 13, 14, 22, 24, 26

EVN European VLBI Network. 7

FAST Five-hundred-meter Aperture Spherical radio Telescope. 27

Fermi GBM Fermi Gamma-ray Burst Monitor. 27

Fermi LAT Fermi Large Area Telescope. 18, 27

FRB Fast Radio Burst. 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27,28

GRB gamma ray burst. 7, 11, 12, 15, 17, 18, 20, 25, 26, 28

GW gravitational wave. 11, 14, 15, 20, 22, 23, 26

HIRAX Hydrogen Intensity and Real Time Analysis Experiment. 27, 28

IGM intergalactic medium. 5, 6, 7, 12, 16

ISM interstellar medium. 19, 24, 26, 27

KBH Kerr black hole. 14, 20

KNBH Kerr-Newman black hole. 14, 15

LGRB long gamma ray burst. 19

LOFAR Low-Frequency Array. 27

LSD large superconducting dipole. 24

MWD magnetic white dwarf. 17

MWN magnetar wind nebula. 18, 19

NS neutron star. 11, 12, 13, 14, 15, 16, 17, 21, 22, 23, 25

31

PBH primordial black hole. 22, 23, 25

PWN pulsar wind nebula. 16, 17

RM rotation measure. 4, 6, 7, 8, 16, 19, 24, 25

SGR soft gamma repeater. 18, 24

sGRB short gamma ray burst. 7, 11, 14, 24

SKA Square Kilometre Array. 27

SLSN I superluminous supernova. 19

SMBH supermassive black hole. 21, 24, 25

SN supernova. 13, 14, 16, 17, 18, 19, 20, 25

SQM strange quark matter. 16

SS strange quark star. 16, 20

Swift/BAT Swift Burst Alert Telescope. 27

UTMOST Molonglo Observatory Synthesis Telescope. 27, 28

WD white dwarf. 12, 13, 22

WH white hole. 25

WSRT Westerbork Synthesis Radio Telescope. 27

32

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