X-Ray Flashes

1. X-Ray Flashes D. Q. Lamb (U. Chicago) HETE-2 Swift “Astrophysical Sources of High-Energy Particles and Radiation” Torun, Poland, 21 June 2005

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/Sγ) > 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 Sakamoto et al. (2005)

5. BeppoSAX HETE Region of Few Bursts Region of No Bursts Slope = 0.5 5 orders of magnitude Dependence of Burst Spectral Peak Energy (Epeak) on Isotropic-Equivalent 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 • Suggest that these three kinds of bursts are closely related phenomena • Key result: approximatelyequal numbers of bursts per logrithmic interval in most observed properties (SE, Eobspeak, Eiso,Epeak, etc.)

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

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 Γ << 300 (Dermer et al. 1999, Huang et al. 2002, Dermer and Mitman 2003). • At the opposite extreme, GRB jets in which the bulk Lorentz factor Γ >> 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).

9. Relation Between Eiso and Einfγ θjet Einfγ = (1-cos θjet) Eiso = ΩjetEiso Eiso = isotropic-equivalent radiated energy Einfγ = inferred radiated energy Uniform Jet

10. Distributions of Eiso and Eγ • Eiso distribution is broad • Einfγ distribution is considerably narrower Ghirlanda, Ghisselini, and Lazzati (2004); see also Frail et al. (2001), Bloom et al. (2003)

11. Universal vs Variable Opening Angle Jets θview = 0o Relativistic Beaming 10o 20o θjet = 20o 40o 60o 40o Universal Jet: Variable Opening Angle (VOA) Jet: Differences due to Differences due to different jet different viewing opening anglesθjet anglesθview

12. Jet Profiles Uniform Jet Gaussian/Fisher Jet Power-Law Jet Rossi, Lazzati, Salmonson, and Ghisellini (2004)

13. Phenomenological Burst Jets

14. Graziani’s Universal Jet Theorem • Universal jet model that produces narrow distribution in one physical quantity (e.g., Einfγ) produces narrow distributions in all other physical quantities (e.g., Epeak, Eiso, etc.) • And vice versa: Universal jet model that produces broad distribution in one physical quantity (e.g., Eiso) produces broad distributions in all other physical quantities (e.g., Epeak, Einfγ, etc.) • But this is not what we observe – what we observe is are broad distributions in Epeak and Eiso, but a relatively narrow distribution in Einfγ • Variable opening angle (VOA) jets can do this because they have an additional degree of freedom: the distribution of jet opening angles θjet

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

16. Uniform Variable Opening-Angle Jet vs. Power-Law Universal Jet DQL, Donaghy, and Graziani (2005) Power-law universal jet Uniform variable opening-angle (VOA) jet

17. Uniform Variable Opening-Angle Jet vs. Power-Law Universal Jet DQL, Donaghy, and Graziani (2005) • VOA uniform jet can account for both XRFs and GRBs • Universal power-law jet can account for GRBs, but not both XRFs and GRBs – because distributions in Eiso and Eobspeak are too narrow

18. Gaussian/Fisher Universal Jet DQL, Donaghy, and Graziani (2005)

19. Phenomenological Burst Jets Favored Disfavored

20. Special Relativistic Beaming • Relativistic beaming produces low Eiso and Epeak values when uniform jet is viewed outside θjet (see Yamazaki et al. 2002, 2003, 2004) • Relativistic beaming must occur • Therefore very faint bursts w. Epeakobs in UV and optical must exist • However, key question is whether relativistic beaming dominates

21. Uniform VOA Jet + Relativistic Beaming Epeak ~ Eiso1/2 Epeak ~ Eiso1/3 Yamazaki, Ioka, and Nakamura (2004)

22. Uniform VOA Jet + Relativistic Beaming Donaghy (2005) Γ = 100 Γ = 300

23. Expected Behavior of Afterglow in Relativistic Beaming Model

24. Observed Behavior of Afterglow • Swift/XRT observations • of XRF 050215b show • that the X-ray afterglow: • Does not show increase followed by rapid decrease • Rather, it joins smoothly onto end of burst • It then fades slowly • Safter/Sburst~ 1 • Jet break time > 5d (> 20d) θjet > 25o (35o) at z = 0.5 Swift: XRF 050215b BeppoSAX: XRF 020427

25. Phenomenological Burst Jets Favored Disfavored Strongly Disfavored

26. 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 XRFs. GRB Spectrum Peaks in Gamma - Rays XRF Spectrum Peaks in X-Rays

27. Conclusions • As most extreme burst population, XRFs provide unique information about structure of GRB jets • Variable opening angle jet models favored; universal jet models disfavored; relativistic beaming models strongly disfavored • Absence of relativistic beaming Γ > 300 • Confirming these conclusions will require • prompt localization of many more XRFs • determination of Epeak • determination of tjet from observations of X-ray afterglows • determination of redshifts z • HETE-2 is ideally suited to do thefirst two, whereas Swift (with Emin ~ 15 keV and 15 keV < E < 150 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 – and therefore all bursts

28. Back Up Slides

29. 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: • Are Einfγ (XRFs) << Einfγ (GRBs)? • Is the XRF population a direct extension of the GRB and X-Ray-Rich GRB populations (e.g., θjet)? • Are XRFs a separate component of GRBs (e.g., core/halo)? • Are XRFs due to different physics than GRBs and X-Ray Rich GRBs (e.g., relativistic beaming)? • Does burst population extend down to UV (and optical)?

30. 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 Γ << 300 (Dermer et al. 1999, Huang et al. 2002, Dermer and Mitman 2003). • At the opposite extreme, GRB jets in which the bulk Lorentz factor Γ >> 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-Γ “core” and XRFs are produced by a low-Γ “halo” (Berger et al. 2004, Huang et al. 2004).

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

33. 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

34. 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

35. Universal Jet Variable Opening Angle (VOA) Jet Rossi, Lazzati, Salmonson, and Ghisellini (2004)

37. Phenomenological Burst Jets

38. Phenomenological Burst Jets

39. Phenomenological Burst Jets

40. Phenomenological Burst Jets

41. Phenomenological Burst Jets

42. Phenomenological Burst Jets

43. Phenomenological Burst Jets Favored

44. 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

45. 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)

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

47. 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

48. 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

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

50. 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