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) gt 0 (i.e., gt 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
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 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) ltlt 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 ltlt 300
  (Dermer et al. 1999, Huang et al. 2002, Dermer
  and Mitman 2003). At opposite extreme, GRB jets
  in which the bulk Gamma gtgt 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)
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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)
HETE-2 bursts
BeppoSAX 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 gt minimum range
  for Omegajet is 6 x 10-5 lt Omegajet lt 6
• Model therefore implies that there are 105 more
  bursts with small Omegajets for every such burst
  we see gt 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
VOA Fisher jet
Universal 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
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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 gt 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)
Even with the BATs 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
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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
• gt 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

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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 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)
• XRF 020903 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/0312634) (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)
• XRFSN Connection
• Possible SN in Afterglow of XRF 030723 (J. P. U.
  Fynbo et al.) ApJ, 609, 962 (2004)
• Model of Possible SN in Afterglow of XRF 030723
  (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 030329 XRF 030723 (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), AA, in press
  (astro-ph/0308297) (2004)
• XRFs from Off-Axis Non-Uniform Jets (Z. P. Jin
  D. M. Wei), AA, submitted (astro-ph/0308061)
  (2004)

34
Ability of HETE-2 and Swift to Measure Epeak and
Sbol of XRFs

• Epeak(estimated) vs. Epeak
• Shaded areas are 68 confidence regions
• Swift (red)
• well-determined for Epeakgt 20 keV
• undetermined for Epeaklt 20 keV
• HETE-2 (blue)
• well-determined down to Epeak 3 keV

Lamb, Graziani, and Sakamoto (2004)

• Sbol(estimated) vs. Sbol
• Shaded areas are 68 confidence regions
• Swift (red)
• well-determined for Epeak gt 20 keV
• undetermined for Epeaklt 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 lt 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 gt
  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 lt 5
  keV
• by factor 3 the number of XRFs w. Epeak lt 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