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/ )

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

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: approximately equal 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 from Lloyd-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. Structured (Core + Halo) Jet Models
Huang et al. (2004)
It is not clear in such a model
why properties of XRFs, XRRs, and GRBs form a continuum
why there are ~ equal numbers of XRFs, XRRs, and GRBs

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

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

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
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

31. Back Up Slides

32. 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

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

34. Ability of HETE-2 and Swift to Measure Epeak and Sbol of XRFs
Lamb, Graziani, and Sakamoto (2004)
Epeak(estimated) vs. Epeak:
Shaded areas are 68% confidence regions
Swift (red):
well-determined for Epeak> 20 keV
undetermined for Epeak< 20 keV
HETE-2 (blue):
well-determined down to Epeak ~ 3 keV
Sbol(estimated) vs. Sbol:
Shaded areas are 68% confidence regions
Swift (red):
well-determined for Epeak > 20 keV
undetermined for Epeak< 20 keV
HETE-2 (blue):
well-determined down to Epeak ~ 3 keV

35. 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
by factor ~ 10 the number of XRFs w. Epeak < 5 keV
by factor ~ 3 the number of XRFs w. Epeak < 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 Epeak for XRFs that Swift XRT and UVOT can follow up – these bursts are crucial for confirming that the Eiso-Epeak relation extends to XRFs