X-Ray Flashes

1. X-Ray Flashes D. Q. Lamb (U. Chicago) 4th Rome GRB Workshop 20 October 2004

2. 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

3. HETE-2 X-Ray Flashes vs. GRBs Sakamoto et al. (2004) GRB Spectrum Peaks in Gamma-Rays XRF Spectrum Peaks in X-Rays

4. 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)

5. 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

6. 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

7. 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)?

8. 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).

9. Observed Eiso Versus Omegajet

10. GRBs Have “Standard” Energies Frail et al. (2001); Kumar and Panaitescu (2001) Bloom et al.(2003)

11. 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

12. Determining If Bursts are Detected DQL, Donaghy, and Graziani (2004) BeppoSAX bursts HETE-2 bursts

13. 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

14. 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

15. 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

16. Universal Gaussian 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

17. 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 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

18. 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

19. Universal Versus VOA Fisher Jets 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)

20. Observed Distribution of Egammainf Berger et al. (2003)

21. Universal Versus VOA Fisher Jet Models Donaghy, Graziani and DQL (2004) – see Poster P-26 Universal Fisher jet VOA Fisher jet

22. 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

23. VOA Uniform Jet + Relativistic Beaming Yamazaki, Ioka, and Nakamura (2004)

24. Uniform Jet + Relativistic Beaming Donaghy (2004) – Poster P-27 Maximum thetajet = 2o Maximum thetajet = 20o

25. Uniform Jet + Relativistic Beaming Donaghy (2004) – Poster P-27 Maximum thetajet = 2o Maximum thetajet = 20o

26. Uniform Jet + Relativistic Beaming Donaghy (2004) – Poster P-27 Maximum thetajet = 2o Maximum thetajet = 20o

27. HETE Passband 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

28. 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 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

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