X-Ray Flashes

X-Ray FlashesX-Ray Flashes

D. Q. Lamb (U. Chicago)

“Astrophysical Sources of High-Energy Particles and Radiation” Torun, Poland, 21 June 2005

HETE-2 Swift

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

HETE-2 X-Ray Flashes vs. GRBs

GRB SpectrumPeaks in Gamma-Rays

XRF Spectrum Peaks in X-Rays

Sakamoto et al. (2004)

Density of HETE-2 Bursts in (Density of HETE-2 Bursts in (SS, , EEpeakpeak)-Plane)-Plane

Sakamoto et al. (2005)

Dependence of Burst Spectral Peak Energy (EDependence of Burst Spectral Peak Energy (Epeakpeak) )

on Isotropic-Equivalent Energy (Eon Isotropic-Equivalent Energy (Eisoiso))

HETE

BeppoSAX

Slope = 0.5

HETE-2 results confirm & extend the Amati et al. (2002)

relation:

Epeak ~ {Eiso} 0.5

Region ofFew Bursts

Region ofNo Bursts

Implications of HETE-2 Observations Implications of HETE-2 Observations of XRFs and X-Ray-Rich GRBsof 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: approximately equal numbers of bursts per logrithmic interval in most observed properties (SE, Eobs

peak, Eiso,Epeak, etc.)

Scientific Importance of XRFsScientific Importance of XRFs

As most extreme burst population, XRFs provide severe constraints on burst models and unique insights intoStructure of GRB jetsGRB rateNature of Type Ic supernovae

Physical Models of XRFsPhysical 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).

Relation Between ERelation Between Eisoiso and E and Einfinfγγ

Uniform Jet

Einfγ = (1-cos θjet) Eiso

= Ωjet Eiso

Eiso = isotropic-equivalent radiated energy

Einfγ = inferred radiated

energy

θjet

Distributions of EDistributions of Eisoiso and E and Eγγ

Ghirlanda, Ghisselini, and Lazzati (2004); see also Frail et al. (2001), Bloom et al. (2003)

Eiso distribution is broad

Einfγ distribution is

considerably narrower

Universal vs Variable Opening Angle JetsUniversal vs Variable Opening Angle Jets

Universal Jet: Variable Opening Angle (VOA) Jet: Differences due to Differences due to different jet different viewing opening angles θjet

angles θview

20o

40oθjet = 20o 40o 60o

θview = 0o Relativistic Beaming10o

Jet ProfilesJet Profiles

Rossi, Lazzati, Salmonson, and Ghisellini (2004)

Uniform Jet Gaussian/Fisher Jet Power-Law Jet

Phenomenological Burst JetsPhenomenological Burst Jets

Jet Profile Jet Opening Angle

Uniform Variable

Gaussian/Fisher Variable

Power-Law Universal

Uniform Universal

Gaussian/Fisher Universal

Uniform Variable + Relativistic Beaming

Gaussian/Fisher Variable + Relativistic Beaming

Power-Law Universal + Relativistic Beaming

Uniform Universal + Relativistic Beaming

Gaussian/Fisher Universal + Relativistic Beaming

Graziani’s Universal Jet TheoremGraziani’s Universal Jet Theorem

Universal jet model that produces narrow distribution in one physical quantity (e.g., Einf

γ) 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, Einf

γ, 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 Einf

γ

Variable opening angle (VOA) jets can do this because they have an additional degree of freedom: the distribution of jet opening angles θjet

Determining If Bursts are Detected

HETE-2 burstsBeppoSAX bursts

DQL, Donaghy, and Graziani (2004)

Uniform Variable Opening-Angle Jet Uniform Variable Opening-Angle Jet vs. Power-Law Universal Jet vs. Power-Law Universal Jet


Power-law universal jet Uniform variable opening-angle (VOA) jet

Uniform Variable Opening-Angle Jet Uniform Variable Opening-Angle Jet vs. Power-Law vs. Power-Law

Universal JetUniversal Jet

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 Eobs

peak are too narrow


Gaussian/Fisher Universal JetGaussian/Fisher Universal Jet


Phenomenological Burst Jets


Uniform Variable


Power-Law Universal

Uniform Universal







Favor

ed

Disfavored

Special Relativistic BeamingSpecial 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

Uniform VOA Jet + Relativistic BeamingUniform VOA Jet + Relativistic Beaming

Yamazaki, Ioka, and Nakamura (2004)

Epeak ~ Eiso1/2

Epeak ~ Eiso1/3

Uniform VOA Jet Uniform VOA Jet + Relativistic Beaming + Relativistic Beaming

Donaghy (2005)

Γ = 100 Γ = 300

Expected Behavior of Afterglow Expected Behavior of Afterglow in Relativistic Beaming Model in Relativistic Beaming Model

Observed Behavior of AfterglowObserved Behavior of Afterglow

Swift: XRF 050215b

BeppoSAX: XRF 020427

Swift/XRT observationsof XRF 050215b showthat 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



Uniform Variable


Power-Law Universal

Uniform Universal






Gaussian/Fisher Universal + Relativistic BeamingStrongly Disfavored

Favor

ed

Disfavored

X-Ray Flashes vs. GRBs: X-Ray Flashes vs. GRBs: HETE-2 and HETE-2 and SwiftSwift (BAT) (BAT)

GRB SpectrumPeaks in Gamma - Rays

XRF Spectrum Peaks in X-Rays

Even with the BAT’s huge

effective area (~2600 cm2), only HETE-2

can determine the spectral

properties of the most XRFs.

ConclusionsConclusions 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 the first two, whereas Swift (with Emin ~ 15 keV and 15 keV < E < 150 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 – and therefore all bursts

Back Up Slides

Scientific Importance of XRFsScientific 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 Einf

γ (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)?

Physical Models of XRFsPhysical 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).

GRBs Have “Standard” Energies

Frail et al. (2001); Kumar and Panaitescu (2001)

Bloom et al.(2003)

Phenomenological Jet Models

Universal ● Power-Law Jet ● Fisher Jet

(Diagram from Lloyd-Ronning and Ramirez-Ruiz 2002)

Variable Opening-Angle (VOA)● Uniform Jet● Fisher Jet

● VOA Uniform Jet + Relativistic Beaming● Core + Halo Jet

Phenomenological Jet Models

Universal ● Power-Law Jet ● Fisher Jet

(Diagram from Lloyd-Ronning and Ramirez-Ruiz 2002)

Variable Opening-Angle (VOA)● Uniform Jet● Fisher Jet

● VOA Uniform Jet + Relativistic Beaming● Core + Halo Jet

Rossi, Lazzati, Salmonson, and Ghisellini (2004)

Universal Jet Variable Opening Angle (VOA) Jet


Uniform Variable


Power-Law Universal

Uniform Universal









Uniform Variable


Power-Law Universal

Uniform Universal









Uniform Variable


Power-Law Universal

Uniform Universal









Uniform Variable


Power-Law Universal

Uniform Universal









Uniform Variable


Power-Law Universal

Uniform Universal









Uniform Variable


Power-Law Universal

Uniform Universal









Uniform Variable


Power-Law Universal

Uniform Universal









Uniform Variable


Power-Law Universal

Uniform Universal







Favor

ed

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

Universal Versus VOA Fisher Jets


VOA Fisher jetUniversal Fisher jet

Peak of Egammainf ~ 5 times smaller than actual value

Egammainf distribution has low-energy tail (of XRFs)

Observed Distribution of Egammainf

Berger et al. (2003)

Universal Gaussian Jet

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

Zhang et al. (2004)

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 Egamma

inf, 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

Universal Versus VOA Fisher Jet Models


Universal Fisher jet VOA Fisher jet

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

Uniform Jet + Relativistic Beaming

Donaghy (2004) – Poster P-27

Maximum thetajet = 2o Maximum thetajet = 20o

Donaghy (2005)

Uniform Variable Opening Angle Jet Uniform Variable Opening Angle Jet + Relativistic Beaming + Relativistic Beaming

Donaghy (2005)

Uniform Variable Opening Angle Jet Uniform Variable Opening Angle Jet + Relativistic Beaming + Relativistic Beaming

Uniform Universal Jet Uniform Universal Jet + Relativistic Beaming+ Relativistic Beaming

Maximum θjet = 2o Maximum θjet = 20o

Donaghy (2005)

Uniform Universal Jet Uniform Universal Jet + Relativistic Beaming+ Relativistic Beaming

Donaghy (2005)

Maximum θjet = 2o Maximum θjet = 20o

Implications of Variable Opening-Angle Uniform Jet

Model implies most bursts have small Ωjet (these bursts are the hardest and most luminous) but we see very few of them

Range in Eiso of five decades => minimum range for Ωjet is ~ 6 x 10-5 < Ωjet < 6

Model therefore implies that there are ~ 105 more bursts with small Ωjet’s for every such burst we see => if so, RGRB might be comparable to RSN

However, efficiency in conversion of Eγ (Ejet) to Eiso

may be less for XRFs, in which case: Minimum opening angle of jet could be larger GRB rate could be smaller

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X-Ray Flashes