1. X-Ray Flashes D. Q. Lamb (U. Chicago) HEAD Meeting, New Orleans, LA 10 September 2004

2. HETE-2 International Science Team Cosmic Radiation Laboratory Institute of Physical and Chemical Research (RIKEN) JAPAN Centre D’Etude Spatiale des Rayonnements (CESR) FRANCE Brazil + India + Italy (Burst Alert Station Scientists) Masaru Matsuoka (NASDA) Nobuyuki Kawai (Tokyo Inst. Tech) Atsumasa Yoshida (Aoyama G. U.) Jean-Luc Atteia Celine Barraud Michel Boer Gilbert Vedrenne Joao Braga Ravi Manchanda Graziella Pizzichini Space Science Laboratory University of California at Berkeley USA Board of Astronomy and Astrophysics University of California at Santa Cruz USA Los Alamos National Laboratory Los Alamos, NM USA Edward E. Fenimore Mark Galassi Kevin Hurley J. Garrett Jernigan Astronomy and Astrophysics Department University of Chicago, IL USA Donald Q. Lamb Jr. (Mission Scientist) Carlo Graziani Tim Donaghy Stanford E. Woosley Donald A.Kniffen (NASA Program Scientist) Scott D.Barthelmy (GSFC Project Scientist ) National Aero & Space Administration USA Center for Space Research Massachusetts Institute of Technology Cambridge, MA USA George R. Ricker (PI) Nat Butler Geoffrey B. Crew John P. Doty Allyn Dullighan Roland K. Vanderspek Joel Villasenor

3. HETE is Going Great Guns…  HETE is currently localizing ~ 25 GRBs yr -1  HETE has localized 68 GRBs in 3.75 yrs of operation (compared to 52 GRBs localized by BeppoSAX during its 6-yr mission)  25 of these localizations have led to the detection of X-ray, optical, or radio afterglows  As of today, redshifts have been reported for 14 of these afterglows  HETE has localized 19 XRFs (compared to 13 for BeppoSAX)  HETE has observed 65 bursts from SGRs 1806-20 and 1900+14 in the summers of 2001- 2004 – and discovered a 6 th SGR: 1808-20  HETE has observed ~ 1000 XRBs

4. GRB020903: Elucidation of “X-ray Flashes” GRB021211: Insight into “Optically Dark” GRB Mystery GRB030329: GRB-SN Connection (SN2003dh; z=0.17) HETE Gamma-ray Bursts: 6 Major Scientific Insights in Past 1.5 Years 6 Major Scientific Insights in Past 1.5 Years GRB020531: First detection of short GRB with prompt optical/X-ray followup GRB020813: X-ray lines from  particle nuclei (Chandra spectra) GRB021004: Refreshed shock or inhomogeneous jet (NASA SSU)

5. “X-Ray Flashes”  Defining “X-ray flashes” (Heise et al. 2000) as bursts for which log (S x /S gamma ) > 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  Nature of XRFs is largely unknown  XRFs may provide unique insights into  Structure of GRB jets  GRB rate  Nature of Type Ic supernovae

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

7. Density of HETE-2 Bursts in (S, E peak )-Plane “Global Properties of XRFs and X-Ray-Rich GRBs Observed by HETE-2,” Sakamoto et al. (2004; astro-ph/0409128)

8. HETE-2 Localizations of XRFs  XRFs have small peak fluxes and fluences; consequently, most XRFs lie below the SXC threshold  Therefore WXM localizations are the key to XRF science w. HETE

9. X-Ray and Optical Afterglows Lamb, Donaghy & Graziani (2004)  X-ray and optical afterglows of XRFs are also faint  Left panel: slope = 0.74 +/- 0.17; right panel: slope = -0.70 +/- 0.15 => tantalizing evidence that efficiency of prompt emission is less for XRFs than for GRBs (as expected from V  L estimator)

10. Dependence of GRB Spectral Peak Energy (E peak ) on Burst Isotropic Radiated Energy (E iso ) HETE BeppoSAX Slope = 0.5 HETE-2 results confirm & extend the Amati et al. (2002) relation: E peak ~ {E iso } 0.5 Region of Few Bursts Region of No Bursts

11. E iso —E peak Relation Within BATSE GRBs Liang & Dai (2004)

12. Dependence of GRB Spectral Peak Energy (E peak ) on Burst Isotropic Radiated Energy (E iso )  GRB 980425 and GRB 031203 are very nearby (z = 0.008 and 0.10)  Both had bright SN light curves  Neither lie on the Amati et al. (2002) relation!  These results strongly suggest:  a different radiation mechanism  a separate population GRB 980425 GRB 031203

13. XRF 020903: Discovery of Optical Afterglow Palomar 48-inch Schmidt images: 2002 Sep 6 (left image), 2002 Sep 28 (middle image), subtracted image (right image) Soderberg et al. (2002)

14. Host Galaxy of XRF 020903 Fruchter et al. (2004) Host galaxy lies at z = 0.25

15. XRF 020903: Implications qHETE-2 and optical follow-up observations of GRB020903 show that this XRF: q Lies on an extension of the (S E,E obs peak )- distribution q Lies on an extension of the Amati et al. (2002) relation q Host galaxy is copiously producing stars, similar to those of GRBs q Host galaxy has a redshift z = 0.25, similar to those of GRBs qThese results provide evidence that GRBs, X-ray-rich GRBs, and X-Ray Flashes are closely related phenomena

16. XRF 030723: Optical Afterglow Fynbo et al. (2004)Tominaga et al. (2004)  Increase at ~ 15 days after burst might be due to SN component – or possibly, jet structure

17. Implications of HETE-2 Observations of XRFs and X-Ray-Rich GRBs qHETE-2 results, when combined with earlier results: q Provide strong evidence that properties of XRFs, X-ray-rich GRBs, and GRBs form a continuum q Key result: approximately equal numbers of bursts per logrithmic interval in all observed properties q Suggest that these three kinds of bursts are closely related phenomena

18. Observations of XRFs Are Stimulating New Theoretical Ideas qXRF & GRB Jet Structure and Burst Rates q A Unified Jet Model of XRFs, X-Ray-Rich GRBs, & GRBs (D. Q. Lamb, T. Q Donaghy & C. Graziani), New Astronomy Reviews, 48, 459 (2004) q Quasi-Universal Gaussian Jets: A Unified Picture for GRBs & XRFs (B. Zhang, X. Dai, N. M. Lloyd-Ronning & P. Meszaros), ApJ, 601, L119 (2004) q XRF 030723: Evidence for a Two-Component Jet (Y. F. Huang, X. F. Wu, Z. G. Dai, H. T. Ma & T. Lu), ApJ, 605, 300 (2004) q XRF 020903: Sub-Luminous & Evidence for A Two-Component Jet (A. Soderberg et al.), ApJ, 606, 994 (2004) q A Unified Jet Model of XRFs, X-Ray-Rich GRBs, & GRBs (D. Q. Lamb, T. Q Donaghy & C. Graziani, ApJ, in press (astro-ph/0312634) (2004) q Unified Model of XRFs, X-Ray-Rich GRBs & GRBs (R. Yamazaki, K. Ioka & T. Nakamura), ApJ, 607, 103 (2004) q Gaussian Universal Jet Model of XRFs & GRBs (X. Dai & B. Zhang), ApJ, submitted (2004) qXRF—SN Connection q Possible SN in Afterglow of XRF 030723 (J. P. U. Fynbo et al.) ApJ, 609, 962 (2004) q Model of Possible SN in Afterglow of XRF 030723 (Tominaga, N., et al.), ApJ, 612,105 (2004) q XRFs & GRBs as a Laboratory for the Study of Type Ic SNe ( (D. Q. Lamb, T. Q Donaghy & C. Graziani), New Astronomy Reviews, in press (2004) q GRB-SN Connection: GRB 030329 & XRF 030723 (J. P. U. Fynbo et al.), Santa Fe GRB Workshop Proceedings, in press (2004) qRelativistic Beaming and Off-Axis Viewing Models of XRFs q Peak Energy-Isotropic Energy Relation in the Off-Axis GRB Model (R. Yamazaki, K. Ioka & T. Nakamura), ApJ, 606, L33 (2004) q Off-Axis Viewing as the Origin of XRFs (S. Ddo, A. Dr & A. De Rujula), A&A, in press (astro-ph/0308297) (2004) q XRFs from Off-Axis Non-Uniform Jets (Z. P. Jin & D. M. Wei), A&A, submitted (astro-ph/0308061) (2004)

19. Origin of GRB Prompt Emission and X-Ray, Optical, and Radio Afterglows  In hydrodynamic picture, prompt emission arises from internal shocks  Afterglows arise from external shock

20. Universal vs. Variable Opening-Angle Jet Models Universal Jet Model Variable Opening-Angle Jet Model (Diagram from Lloyd-Ronning and Ramirez-Ruiz 2002)

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

22. Comparison of Variable Opening-Angle and Power-Law Universal Jet Models DQL, Donaghy, and Graziani (2004) Variable Opening-Angle Jet Model Power-Law Universal Jet Model

23. Comparison of Universal and Uniform Jet Models  Uniform jet model can account for both XRFs and GRBs  Power-law universal jet model can account for GRBs, but not both XRFs and GRBs DQL, Donaghy, and Graziani (2004)

24. Implications of Variable Opening-Angle Jet Model qModel implies most bursts have small Omega jet (these bursts are the hardest and most luminous) but we see very few of them qRange in E iso of five decades => minimum range for Omega jet is ~ 6 x 10 -5 < Omega jet < 6 qModel therefore implies that there are ~ 10 5 more bursts with small Omega jet ’s for every such burst we see => if so, R GRB may be comparable to R SN qHowever, efficiency in conversion of E gamma (E jet ) to E iso may be less for XRFs, in which case: q Minimum opening angle of jet could be larger q GRB rate could be smaller

25. Gaussian Universal Jet Model Zhang et al. (2004)  Gaussian universal jet model:  can produce ~ equal numbers of bursts per logarithmic interval  appears to require minimum theta jet ~ 1 o, like the variable opening-angle jet model

26. X-Ray Flashes vs. GRBs: HETE-2 and Swift (BAT) GRB Spectrum Peaks in Gamma - Rays XRF Spectrum Peaks in X-Rays Even with the BAT’s huge effective area (~2600 cm 2 ), only HETE-2 can determine the spectral properties of the most extreme half of XRFs.

27. Ability of HETE-2 and Swift to Measure E peak and S bol of XRFs E peak (estimated) vs. E peak : qShaded areas are 68% confidence regions qSwift (red): q well-determined for E peak > 20 keV q undetermined for E peak < 20 keV qHETE-2 (blue): q well-determined down to E peak ~ 3 keV S bol (estimated) vs. S bol :  Shaded areas are 68% confidence regions  Swift (red):  well-determined for E peak > 20 keV  undetermined for E peak < 20 keV  HETE-2 (blue):  well-determined down to E peak ~ 3 keV Lamb, Graziani, and Sakamoto (2004)

28. HETE-2 Bursts in (S, E peak )-Plane Sakamoto et al. (2004) Redshifts missing

29. HETE-2 Synergies with Swift  HETE-2 can ~ double number of very bright GRBs at z < 0.5 that Swift XRT and UVOT can follow up – these bursts are crucial for understanding the GRB – SNe connection  HETE-2 can ~ double number of bright GRBs at z > 5 that Swift XRT and UVOT can follow up – these bursts are crucial probes of the very high-z universe  HETE-2 can increase q by factor ~ 10 the number of XRFs w. E peak < 5 keV q by factor ~ 3 the number of XRFs w. E peak < 10 keV that Swift can follow up for X-ray & optical afterglows – these bursts are crucial for determining the nature of XRFs, structure of GRB jets, GRB rate, relationship between GRBs and Type Ic SNe  HETE-2 can provide bolometric S and E peak for XRFs that Swift XRT and UVOT can follow up – these bursts are crucial for confirming that the E iso -E peak relation extends to XRFs

30. Conclusions  HETE-2 has provided strong evidence that XRFs, “X-ray-rich” GRBs, and GRBs are closely related phenomena  XRFs provide unique information about q structure of GRB jets q GRB rate q nature of Type Ic SNe qExtracting this information will require prompt q localization of many XRFs q determination of E peak q identification of X-ray and optical afterglows q determination of redshifts qHETE-2 is ideally suited to do the first two, whereas Swift (with E min ~ 15 keV) is not; Swift is ideally suited to do the second two, whereas HETE-2 cannot qPrompt Swift XRT and UVOT observations of HETE-2 XRFs can therefore greatly advance our understanding of XRFs