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![Page 1: Radio Searches of GW Counterparts Current and future capabilities Dale A. Frail National Radio Astronomy Observatory.](https://reader036.fdocuments.net/reader036/viewer/2022062321/56649e725503460f94b720ae/html5/thumbnails/1.jpg)
Radio Searches of GW CounterpartsCurrent and future capabilities
Dale A. FrailNational Radio Astronomy Observatory
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Talk outline.
• What is the expected strength of the radio signal?– Afterglow component. Early and Late. (robust)
– Prompt counterpart (speculative).
• How do we detect the radio signal of a GW trigger?– The quiescent and transient radio sky. A primer.
– Current and future radio facilities.
– Three search strategies (in order of probability of success)
• What follow-up would we want to do?
• What can we be doing today to help the field?
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νm≈Γ4 (i.e radio AG traces trans-relativistic ejecta)
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Afterglow Radio Signal – Robust• Early radio emission (~days, weeks)
– SHB have lower average redshifts, energy and circumburst densities compared to long duration GRBs
– Only two SHB detected in radio out of ~25 Swift events.
GRB 050724 (z=0.257) and GRB 051221 (z=0.546)
– Best estimate is <Fradio>=100 μJy and <z>=0.5
– Predicts 10’s mJy at 1-10 GHz for d=200 Mpc
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Afterglow Radio Signal – Robust
Van Eerten et al. (2010).
Early, on-axis
Early, on-axis
Late, off-axisLate, off-axis
L-GRB
Late-time radio detects AG independent of beaming
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Afterglow Radio Signal – Robust• Early radio emission (~days, weeks)
– SHB have lower average redshifts, energy and circumburst densities compared to long duration GRBs
– Only two SHB detected in radio out of ~25 Swift events.
GRB 050724 (z=0.257) and GRB 051221 (z=0.546)
– Best estimate is <Fradio>=100 μJy and <z>=0.5
– Predicts 10’s mJy at 1-10 GHz for d=200 Mpc
• Late-time radio emission (~months)– Outflow expands, becomes quasi-isotropic and non-
relativistic. A late-time radio turn on independent of original jet direction.
– For reasonable SHB parameters t=30 days, F=0.3 mJy at 1.4 GHz at 300 Mpc (Nakar et al. in prep)
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Prompt Radio Signal – Speculative
• Gravitationally excited MHD waves (Postnov & Pshirkov 2009)
– Predicts 12.5 Kilo-Jy at 100 MHz for d=200 Mpc
• Rotational energy of post-merger object (Moortgat & Kuijpers 2004)
– Predicts 50 Mega-Jy at 30 MHz for d=200 Mpc
• Emission from PSR-like magnetosphere (Hansen & Lyutikov 2001)– Predicts 1 milli-Jy at 400 MHz for d=200 Mpc
• “Back of the envelope” approach– Radio emission is seen in all high energy processes where there
are relativistic particles and magnetic fields– Assume that 10-6 of energy of a SHB goes into a prompt radio
signal– Average fluence for SHB is 10-6 erg cm-2. Duration 0.1 s– Predicts 1 kilo-Jy at 1 GHz
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Quiescent and Transient Radio Sky. Primer.• Isotropic source distribution on sky
– Above 1 mJy source populations are AGN dominated – Below 1 mJy star-formation galaxies start to emerge
• The transient radio sky is quiet– GHz flux density range 0.1 mJy to 10 Jy is well studied
by several (heterogeneous surveys)– Transients are 10-3 to 10-4 of quiescent population
• e.g. Levinson et al. NVSS/FIRST comparison• Ofek et al. survey
• Important implication is that radio false EM-GW detection rate will be small (<0.1 deg2 at 1 mJy)
… and any background events are likely to be AGN, and hence easily filtered out.
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The Quiescent Radio Sky is Isotropic
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J. Condon
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Quiescent and Transient Radio Sky• Isotropic source distribution on sky
– Above 1 mJy source populations are AGN dominated – Below 1 mJy star-formation galaxies start to emerge
• The transient radio sky is quiet– GHz flux density range 0.1 mJy to 10 Jy is well studied
by several (heterogeneous surveys)– Transients are 10-3 to 10-4 of quiescent population
• e.g. Levinson et al. NVSS/FIRST comparison• Ofek et al. survey
• Important implication is that radio false EM-GW detection rate will be small (<0.1 deg2 at 1 mJy)
… and any background events are likely to be AGN, and hence easily filtered out.
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The Transient Radio Sky is Quiet
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Ofek et al. (2011)
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Quiescent and Transient Radio Sky• Isotropic source distribution on sky
– Above 1 mJy source populations are AGN dominated – Below 1 mJy star-formation galaxies start to emerge
• The transient radio sky is quiet– GHz flux density range 0.1 mJy to 10 Jy is well studied
by several (heterogeneous surveys)– Transients are 10-3 to 10-4 of quiescent population
• e.g. Levinson et al. NVSS/FIRST comparison• Ofek et al. survey
• Important implication is that radio false EM-GW detection rate will be small (<0.1 deg2 at 1 mJy)
… and any background events are likely to be AGN, and hence easily filtered out.
![Page 13: Radio Searches of GW Counterparts Current and future capabilities Dale A. Frail National Radio Astronomy Observatory.](https://reader036.fdocuments.net/reader036/viewer/2022062321/56649e725503460f94b720ae/html5/thumbnails/13.jpg)
Radio facilities for GW-EM Counterpart Searches: 2011 and Beyond
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EVLA
WSRT/Apertif
LOFAR
ASKAP
MWAMeerKAT
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Radio facilities for GW-EM Counterpart Searches
RadioFacility
ObservingFreq.
Field of View
1 hr rms
Beam StartDate
ASKAP 1.4 GHz 30 deg2 30 uJy 20” 2013
Apertif 1.4 GHz 8 deg2 50 uJy 15” 2013
MeerKAT
1.4 GHz 1.5 deg2 35 uJy 15” 2013
EVLA 1.4 GHz 0.25 deg2 7 uJy 1.3-45” 2010
EVLA 327 MHz 5 deg2 2 mJy 5-18” 2011
LOFAR 110-240 MHz 50 deg2 1 mJy 5” 2011
EVLA 74 MHz 100 deg2 50 mJy 25-80” 2011
MWA 80-300 MHz 1000 deg2 8 mJy 300” 2011+
LOFAR 15-80 MHz 500 deg2 8 mJy 120” 2011
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(Only Apertif, EVLA, LOFAR has demonstrated noise perfprmance)
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Radio facilities for GW-EM Counterpart Searches: ASCAP
• Australian-lead effort• 36 12-m antennas• Operates at 1.4 GHz• Focal-plane array technology
to give 30 deg2 FoV• 1-hrs, rms~30 uJy (claimed)• 75% of the time given to Key
Science Projects (25% open)– Continuum sky survey 40X
deeper than NVSS– Slow and fast transient
searches
• 2013 delivery (optimistic)
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Radio facilities for GW-EM Counterpart Searches: Apertif
• Dutch effort• Upgrade of WSRT using
FPAs• 14 25-m antennas• Demonstrated peformance• Operates at 1.4 GHz• 8 deg2 FoV• 1-hrs, rms~50 uJy • 75% of the time will be
given to Key Science Projects (25% open)– Proposals in April 2011
• 2013 operation
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Radio facilities for GW-EM Counterpart Searches: MeerKAT
• South African-lead effort• 80 12-m antennas• Operates 0.9-1.75 GHz.
Expansion plans 8-14.5 GHz and 0.58-2.56 GHz
• Focal-plane array technology to give 1.5 deg2 FoV
• 1-hrs, rms~35 uJy (claimed)• 75% of the time given to Key
Science Projects (25% open)– Continuum sky survey – Slow and fast transient
searches
• 2013 delivery of 1.4 GHz
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Radio facilities for GW-EM Counterpart Searches: EVLA
• The 500-lb gorilla of radio astronomy
• 27 25-m antennas• Upgrade project almost
finished. Will deliver order of magnitude increase in continuum sensitivity
• 1-50 GHz + 74 and 327 MHz
• 1-hrs, rms~7 uJy at 1.4 GHz• Responds to external
triggers• Sub-arrays can be used to
image a large error box
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Radio facilities for GW-EM Counterpart Searches: EVLA
• The 500-lb gorilla of radio astronomy
• 27 25-m antennas• Upgrade project almost
finished. Will deliver order of magnitude increase in continuum sensitivity
• 1-50 GHz + 74 and 327 MHz• 1-hrs, rms~7 uJy at 1.4 GHz• Responds to external
triggers• Sub-arrays can be used to
image a large (irregular) error box
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Radio facilities for GW-EM Counterpart Searches: LOFAR
• Dutch-lead European project• 36 Dutch stations, 8 Euro
stations• 15-80 MHz & 110-240 MHz• Key Science Projects
– Continuum sky survey– Slow and fast transient
searches
• Real-time pipeline + alert system and external triggers all planned
• RSM will monitor 25% of sky• Million source survey in 2011
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Radio sky monitor (RSM)
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How might we best detect radio signals?Three strategies in order of chance of success
– Afterglow search at late times for off-axis emission• 0.1 to 1 mJy• Timescales of a month• EVLA, ASKAP, MerrKAT, Apertif
– Afterglow search for on-axis event • Bright but rare (i.e. beamed) 1-10 mJy• Timescales of days• EVLA, ASKAP, MerrKAT, Apertif
– Search for prompt signal• 1 mJy to 1 MJy (i.e. highly uncertain)• Low frequency arrays. LOFAR, MWA, electronically steered
in response to GW trigger• Signal will be dispersively delayed
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How might we best detect prompt signal?• Prompt signal will suffer
dispersive delay and scattering
• Sources of dispersive delay
– Our Galaxy, IGM, host galaxy and circumburst medium
• Expect DM=1000 pc cm-3, or delays of 13 min at 75 MHz
• Dispersive delay scales as ν-2
• Scattering effects (due to turbulence) are more difficult of estimate.
– 0.1 to 4 s at 75 MHz
– Scattering scales as ν-4.4
DM (pc cm-3)
Lorimer and
Kram
er (2
005)
€
τDM ∝ δν
ν 3
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How might we best detect prompt signal?• Prompt signal will suffer
dispersive delay and scattering
• Sources of dispersive delay
– Our Galaxy, IGM, host galaxy and circumburst medium
• Expect DM=1000 pc cm-3, or delays of 13 min at 75 MHz
• Dispersive delay scales as ν-2
• Scattering effects (due to turbulence) are more difficult of estimate.
– 0.1 to 4 s at 75 MHz
– Scattering scales as ν-4.4
DM (pc cm-3)
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What follow-up would we want to do?• Panchromatic modeling to
derive real estimates of energy and circumburst density.
• Direct VLBI imaging of the relativistic shock from the afterglow and any radio-emitting detritus from the merger
• Sub-milliarcsecond resolution
• An simple VLBA imaging project. Easier than GRB 030329 (z=0.17)
• Rule of thumb: If LIGO can detect a merger, the VLBA can image it.
GRB 030329z=0.17 (800 Mpc)Pihlstrom et al. (2007)
1 mas at 100 Mpc is 0.5 pc
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What follow-up would we want to do?• Panchromatic modeling to
derive real estimates of energy and circumburst density.
• Direct VLBI imaging of the relativistic shock from the afterglow and any radio-emitting detritus from the merger
• Sub-milliarcsecond resolution
• An simple VLBA imaging project. Easier than GRB 030329 (z=0.17)
• Rule of thumb: If LIGO can detect a merger, the VLBA can image it.
GRB 030329z=0.17Pihlstrom et al. (2007)
1 mas at 100 Mpc is 0.5 pc Bietenholz et al. (2003)
SNe 1993J at d=4 Mpc
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What follow-up would we want to do?• Panchromatic modeling to
derive real estimates of energy and circumburst density.
• Direct VLBI imaging of the relativistic shock from the afterglow and any radio-emitting detritus from the merger
• Sub-milliarcsecond resolution
• An simple VLBA imaging project. Easier than GRB 030329 (z=0.17)
• Rule of thumb: If LIGO can detect a merger, the VLBA can image it.
GRB 030329z=0.17Pihlstrom et al. (2007)
1 mas at 100 Mpc is 0.5 pc Bietenholz et al. (2003)
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What can we be doing today to help field?• Continue to study GW populations
– AM CVn stars– Core collapse (relativistic) SNe– Short-hard bursts
• Characterize the quiescent and transient radio sky to flux densities of 10 uJy
• Develop robust systems to respond to external triggers– Capability to carry out real-time response of radio
telescopes to transients is rare
– Nasu radio transients are an interesting test case. Bright, short lived with poor localization.
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Conclusions
• Radio counterpart searches are a powerful tool– Predict a bright signal 1-10 mJy
– Independent of beaming
– Short latency is not needed. (Mañana!)
– False positives are relatively unimportant
• A “bonanza” of new radio facilities is coming on line at just the right times for the next generation GW detectors
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The future looks brightCome and join the GW-EM adventure
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