X-Ray Flashes

D. Q. Lamb (U. Chicago)

4th Rome GRB Workshop 20 October 2004

X-Ray Flashes

X-Ray Flashes discovered by Heise et al. (2000) using WFC on BeppoSAX

Defining X-ray flashes as bursts for which log (Sx/Sgamma) > 0 (i.e., > 30 times that for “normal” GRBs) ~ 1/3 of bursts localized by

HETE-2 are XRFs ~ 1/3 are “X-ray-rich” GRBs

(“XRRs”)

Nature of XRFs is still largely unknown

HETE-2 X-Ray Flashes vs. GRBs

GRB SpectrumPeaks in Gamma-Rays

XRF Spectrum Peaks in X-Rays

Sakamoto et al. (2004)

Density of HETE-2 Bursts in (S, Epeak)-Plane

“Global Properties of XRFs and X-Ray-Rich GRBs Observed by HETE-2,” Sakamoto et al. (2004; astro-ph/0409128)

Dependence of GRB Spectral Peak Energy (EDependence of GRB Spectral Peak Energy (Epeakpeak) on ) on

Burst Isotropic Radiated Energy (EBurst Isotropic Radiated Energy (Eisoiso))

HETE

BeppoSAX

Slope = 0.5

HETE-2 results confirm & extend the Amati et al. (2002)

relation:

Epeak ~ {Eiso} 0.5

Region ofFew Bursts

Region ofNo Bursts

Implications of HETE-2 Observations of XRFs and X-Ray-Rich GRBs

HETE-2 results, when combined with earlier BeppoSax and optical follow-up results:Provide strong evidence that properties of XRFs,

X-ray-rich GRBs (“XRRs”), and GRBs form a continuum

Key result: approximately equal numbers of bursts per logrithmic interval in all observed properties

Suggest that these three kinds of bursts are closely related phenomena

Scientific Importance of XRFs

As most extreme burst population, XRFs provide severe constraints on burst models and unique insights into Structure of GRB jets GRB rate Nature of Type Ic supernovae

Some key questions regarding XRFs: Is Egamma (XRFs) << Egamma (GRBs)?

Is the XRF population a direct extension of the GRB and X-Ray-Rich GRB populations?

Are XRFS a separate component of GRBs? Are XRFs due to different physics than GRBs

and X-Ray Rich GRBs? Does burst population extend down to UV (and optical)?

Physical Models of XRFs

X-ray photons may be produced by the hot cocoon surrounding the GRB jet as it breaks out and could produce XRF-like events if viewed well off axis of jet (Meszaros et al. 2002, Woosley et al. 2003).

“Dirty fireball” model of XRFs posits that baryonic material is entrained in the GRB jet, resulting in a bulk Lorentz factor Gamma << 300 (Dermer et al. 1999, Huang et al. 2002, Dermer and Mitman 2003). At opposite extreme, GRB jets in which the bulk Gamma >> 300 and the contrast between the bulk Lorentz factors of the colliding relativistic shells are small can also produce XRF-like events (Mochkovitch et al. 2003).

A highly collimated GRB jet viewed well off the axis of the jet will have low values of Eiso and Epeak because of the effects of relativistic beaming (Yamazaki et al. 2002, 2003, 2004).

XRFs might be produced by a two-component jet in which GRBs and XRRs are produced by a high-Gamma core and XRFs are produced by a low-Gamma “halo” (Huang et al. 2004).

Observed Eiso Versus Omegajet

GRBs Have “Standard” Energies

Frail et al. (2001); Kumar and Panaitescu (2001)

Bloom et al.(2003)

Phenomenological Jet Models

Universal ● Power-Law Jet ● Fisher Jet

(Diagram from Lloyd-Ronning and Ramirez-Ruiz 2002)

Variable Opening-Angle (VOA)● Uniform Jet● Fisher Jet

● VOA Uniform Jet + Relativistic Beaming● Core + Halo Jet

Determining If Bursts are Detected

HETE-2 burstsBeppoSAX bursts

DQL, Donaghy, and Graziani (2004)

Variable Opening-Angle Uniform Jet Versus Universal Power-Law Jet

DQL, Donaghy, and Graziani (2004)

Variable opening-angle (VOA) Universal power-law jet uniform jet

Variable Opening-Angle Uniform Jet Versus Universal

Power-Law Jet

VOA uniform jet can account for both XRFs and GRBs Universal power-law jet can account for GRBs, but not both XRFs and GRBs

DQL, Donaghy, and Graziani (2004)

Implications of Variable Opening-Angle Uniform Jet

Model implies most bursts have small Omegajet (these bursts are the hardest and most luminous) but we see very few of them

Range in Eiso of five decades => minimum range for Omegajet is ~ 6 x 10-5 < Omegajet < 6

Model therefore implies that there are ~ 105 more bursts with small Omegajet’s for every such burst we see => if so, RGRB might be comparable to RSN

However, efficiency in conversion of Egamma (Ejet) to Eiso

may be less for XRFs, in which case: Minimum opening angle of jet could be larger GRB rate could be smaller

Universal Gaussian Jet

In response to conclusion of DQL, Donaghy, and Graziani (2004), Zhang et al. (2004) proposed universal Gaussian jet Universal Gaussian jet

can produce ~ equal numbers of bursts per logarithmic interval requires minimum thetajet ~ 2o as does VOA uniform jet

Zhang et al. (2004)

Fisher Jet Models

We have shown mathematically that universal jet with emissivity given by Fisher distribution (which is natural extension of Gaussian distribution to sphere) have unique property of producing equal numbers of bursts per logarithmic interval in Eiso and therefore in most burst properties (Donaghy, Graziani, and DQL 2004 – Poster P-26)

We have also shown that Fisher jet produces a broad distribution in inferred radiated gamma-ray energy Egamma

inf, in contrast with VOA uniform jet We have simulated universal and VOA Fisher jets We find – as expected – that both models can

reproduce most burst properties However, both models require minimum thetajet ~ 2o,

similar to VOA uniform jet

Universal Versus VOA Fisher Jets

Donaghy, Graziani and DQL (2004) – see Poster P-26

Universal Fisher jet w.minimum thetajet = 2o

VOA Fisher jet w.minimum thetajet = 2o

Universal Versus VOA Fisher Jets

Donaghy, Graziani and DQL (2004) – see Poster P-26

VOA Fisher jetUniversal Fisher jet

Peak of Egammainf ~ 5 times smaller than actual value

Egammainf distribution has low-energy tail (of XRFs)

Observed Distribution of Egammainf

Berger et al. (2003)

Universal Versus VOA Fisher Jet Models

Donaghy, Graziani and DQL (2004) – see Poster P-26

Universal Fisher jet VOA Fisher jet

VOA Uniform Jet + Relativistic Beaming

Relativistic beaming produces low Eiso and Epeak values when uniform jet is viewed outside thetajet (see Yamazaki et al. 2002, 2003, 2004) Relativistic beaming must be present

Therefore very faint bursts w. Epeakobs in UV

and optical must exist However, key question is whether this effect dominates Yamazaki et al. (2004) use VOA uniform jet for XRRs and GRBs, relativistic beaming for XRFs If Gamma ~ 100, some XRFs produced by relativistic beaming are detectable; but if Gamma ~ 300, very few are detectable => difficult to produce ~ equal numbers of XRFs, XRRs, and GRBs

VOA Uniform Jet + Relativistic Beaming

Yamazaki, Ioka, and Nakamura (2004)

Uniform Jet + Relativistic Beaming

Donaghy (2004) – Poster P-27

Maximum thetajet = 2o Maximum thetajet = 20o

Uniform Jet + Relativistic Beaming

Donaghy (2004) – Poster P-27

Maximum thetajet = 2o Maximum thetajet = 20o

Uniform Jet + Relativistic Beaming

Donaghy (2004) – Poster P-27

Maximum thetajet = 2o Maximum thetajet = 20o

X-Ray Flashes vs. GRBs: X-Ray Flashes vs. GRBs: HETE-2 and HETE-2 and SwiftSwift (BAT) (BAT)

GRB SpectrumPeaks in Gamma - Rays

XRF Spectrum Peaks in X-Rays

Even with the BAT’s huge

effective area (~2600 cm2), only HETE-2

can determine the spectral

properties of the most extreme half of XRFs.

Conclusions HETE-2 has provided strong evidence that XRFs, “X-ray-rich”

GRBs, and GRBs are closely related phenomena XRFs provide unique information about

structure of GRB jets GRB rate nature of Type Ic SNe

Extracting this information will require prompt localization of many XRFs determination of Epeak

identification of X-ray and optical afterglows determination of redshifts

HETE-2 is ideally suited to do the first two, whereas Swift (with Emin ~ 15 keV) is not; Swift is ideally suited to do the second two, whereas HETE-2 cannot

Prompt Swift XRT and UVOT observations of HETE-2 XRFs can therefore greatly advance our understanding of XRFs

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