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X-Ray Flashes. D. Q. Lamb (U. Chicago). 4 th Rome GRB Workshop 20 October 2004. X-Ray Flashes. X-Ray Flashes discovered by Heise et al. (2000) using WFC on Beppo SAX Defining X-ray flashes as bursts for which log ( S x / S gamma ) > 0 (i.e., > 30 times that for “normal” GRBs)

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X ray flashes
X-Ray Flashes

D. Q. Lamb (U. Chicago)

4th Rome GRB Workshop

20 October 2004


X ray flashes1
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
HETE-2 X-Ray Flashes vs. GRBs

Sakamoto et al. (2004)

GRB Spectrum

Peaks in Gamma-Rays

XRF Spectrum

Peaks in X-Rays


Density of hete 2 bursts in s e peak plane
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 e peak on burst isotropic radiated energy e iso

BeppoSAX

HETE

Region of

Few Bursts

Region of

No Bursts

Slope = 0.5

5 orders of magnitude

Dependence of GRB Spectral Peak Energy (Epeak) on Burst Isotropic Radiated Energy (Eiso)

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

Epeak ~ {Eiso} 0.5


Implications of hete 2 observations of xrfs and x ray rich grbs
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: approximatelyequal 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
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
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 e iso versus omega jet
Observed Eiso Versus Omegajet


Grbs have standard energies
GRBs Have “Standard” Energies

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

Bloom et al.(2003)


Phenomenological jet models
Phenomenological Jet Models

(Diagram fromLloyd-Ronning and Ramirez-Ruiz 2002)

Universal

  • Power-Law Jet

  • Fisher Jet

  • Variable Opening-Angle (VOA)

  • Uniform Jet

  • Fisher Jet

  • VOA Uniform Jet + Relativistic Beaming

  • Core + Halo Jet


Determining if bursts are detected
Determining If Bursts are Detected

DQL, Donaghy, and Graziani (2004)

BeppoSAX bursts

HETE-2 bursts


Variable opening angle uniform jet versus universal power law jet
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 jet1
Variable Opening-Angle Uniform Jet Versus Universal Power-Law Jet

DQL, Donaghy, and Graziani (2004)

  • VOA uniform jet can account for both XRFs and GRBs

  • Universal power-law jet can account for GRBs, but not

    both XRFs and GRBs


Implications of variable opening angle uniform jet
Implications of Variable Opening-Angle Uniform Jet Versus Universal Power-Law 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
Universal Gaussian Jet Versus Universal Power-Law Jet

Zhang et al. (2004)

  • 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


Fisher jet models
Fisher Jet Models Versus Universal Power-Law Jet

  • 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 Egammainf, 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
Universal Versus VOA Fisher Jets Versus Universal Power-Law Jet

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 jets1
Universal Versus VOA Fisher Jets Versus Universal Power-Law Jet

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

Universal Fisher jet

VOA Fisher jet

  • Peak of Egammainf ~ 5 times smaller than actual value

  • Egammainf distribution has low-energy tail (of XRFs)


Observed distribution of e gamma inf
Observed Distribution of E Versus Universal Power-Law Jetgammainf

Berger et al. (2003)


Universal versus voa fisher jet models
Universal Versus VOA Fisher Jet Models Versus Universal Power-Law Jet

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

Universal Fisher jet

VOA Fisher jet


Voa uniform jet relativistic beaming
VOA Uniform Jet + Relativistic Beaming Versus Universal Power-Law Jet

  • 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 beaming1
VOA Uniform Jet + Relativistic Beaming Versus Universal Power-Law Jet

Yamazaki, Ioka, and Nakamura (2004)


Uniform jet relativistic beaming
Uniform Jet + Relativistic Beaming Versus Universal Power-Law Jet

Donaghy (2004) – Poster P-27

Maximum thetajet = 2o

Maximum thetajet = 20o


Uniform jet relativistic beaming1
Uniform Jet + Relativistic Beaming Versus Universal Power-Law Jet

Donaghy (2004) – Poster P-27

Maximum thetajet = 2o

Maximum thetajet = 20o


Uniform jet relativistic beaming2
Uniform Jet + Relativistic Beaming Versus Universal Power-Law Jet

Donaghy (2004) – Poster P-27

Maximum thetajet = 2o

Maximum thetajet = 20o


X ray flashes vs grbs hete 2 and swift bat

HETE Passband Versus Universal Power-Law Jet

Swift Passband

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

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.

GRB Spectrum

Peaks in Gamma - Rays

XRF Spectrum

Peaks in X-Rays


Conclusions
Conclusions Versus Universal Power-Law Jet

  • 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 thefirst two, whereas Swift (with Emin ~ 15 keV) is not; Swift is ideally suited to do thesecond two, whereas HETE-2 cannot

  • Prompt Swift XRT and UVOT observationsof HETE-2 XRFscan therefore greatlyadvance our understanding of XRFs


Back up slides
Back Up Slides Versus Universal Power-Law Jet


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