Global Properties of X-ray Afterglows Observed with XRT

ENWEI LIANGENWEI LIANG （梁恩维）（梁恩维）University of Guangxi, Nanning University of Guangxi, Nanning

astro.gxu.edu.cn astro.gxu.edu.cn Nanjing 2008.06.23Nanjing 2008.06.23

Collaborators

BING ZHANG BING ZHANG (UNLV)(UNLV)

BINBIN ZHANG BINBIN ZHANG (UNLV)(UNLV)

Judith L. RacusinJudith L. Racusin (PENN. STATE UNIVERSITYPENN. STATE UNIVERSITY)

DAVE BURROWS DAVE BURROWS (PENN. STATE UNIVERSITY)(PENN. STATE UNIVERSITY)

Outline

General features of Swift/XRT LCs Our Data Reduction Code Results of our comprehensive analysis

– Shallow decay segment– Normal decay segment– Jet-like decay segment

Summary

~ 90 % Swift GRBs have X-ray afterglow detection.

1.General features of XRT LCs(see Dave’s talk this morning)

Burrows et al. 2005; Zhang et al. 2006;

Nousek et al. 2006; O’Brien et al. 2006

Canonical XRT LC X-ray Flares Non-Canonical XRT LC

100 1000 10000 1000001E-14

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

061007

Flu

x (

erg

cm-2 s

-1)

Time (s)

080319B

Lv et al. 2008( in preparation)

BinBin’s talk This talk

Guido’s Talk

2.Our Data Reduction Code

Setting up two data processing servers at UNLV & GXU Full Function for Swift BAT & XRT data process:

– automatically download and maintain Swift data– Automatically make pileup correction and exposure correctio

n– Extract the BAT and XRT lightcurves at any user-specified t

emporal and spectral regimes– Automatically extract the BAT and XRT spectra and fit with X

spec– Output eps figures for publication

Scripts and data are available at : – http://grb.physics.unlv.edu/ ( @ UNLV)– http://astro.gxu.edu.cn/web/ ( @ GXU)

3.What we did: A comprehensive analysis

Data: two-year XRT observations.

Temporal analysis Spectral analysis Data vs Fireball model: Cha

llenges to conventional GRB models

1.LC fitting model:

Broken power law

2. Sample selection Criterion:

No significant flares in the afterglow phase (reduced chi^2<2).

Both the shallow and normal decay segments are bright enough to make spectral analysis.

3. Sample: 52 bursts from 2 year XRT data (179)

4. ShallowNormal decay segments:(1)Data & Analysis method

Zhang et al. 2006

(Liang et al. 2007, ApJ)

4. ShallowNormal decay segments:(2)Distributions of Characteristcs

(Liang et al. 2007, ApJ)

Break time X-ray fluence

Ph. Index Decay slope

4. ShallowNormal decay segments:(3) Correlations with Prompt Emisison

(Liang et al. 2007, ApJ)

Duration

Ph. Index

fluence

Energy

4. ShallowNormal decay segments: (4)spectral evolution!

(Liang et al. 2007, ApJ)

4. ShallowNormal decay segments: (5) Data vs closure relations

Most of the normal decay segments are roughly consistent with the closure relationsEnergy Injection?Four cases are notInternal origin?.

(Liang et al. 2007, ApJ)

4. ShallowNormal decay segments: (6) Energy injection?

Liang-Zhang relation

(Liang et al. 2007)

Eiso t∝ b,opt-1.2

4. ShallowNormal decay segments: (6) X-Ray vs Optical (a)

(Liang et al. 2007)

Achromatic!

4. ShallowNormal decay segments: (6) X-Ray vs Optical (b)

(Liang et al. 2007)

Chromatic!

A Great issue for the energy injection scenario!

Continuous energy injection? (Rees & Meszaros 1998; Dai & Lu 1998; Panaitescu et al. 1998; Sari & Meszaros 2000 ; Zhang Meszaros et al. 2001; Wang & Dai 2001; Dai 2004; Granot & Kumar 2006; Panaitescu 2007; Yu \& Dai 2007; Liang et al. 2007b)Delayed onset of the afterglow emission? (Kobayashi \& Zhang 2007)Off-beam jet effect? (Toma et al. 2006; Eichler & Granot 2006)pre-cursor?(Ioka et al. 2006);Two-component jet ? (Granot et al. 2006;Jin et al. 2007),Varying microphysics parameter?(Ioka et al. 2006; Panaitescu et al. 2006b; Fan & Piran 2006; Granot et al. 2006) Reflection? (Panaitescu et al. ; Shen et al. 2007) Dust scattering of prompt X-rays ?(Shao & Dai 2007) Emission from a long-lived reverse shock? (Uhm & Beloborodov 2007; Genet, Daigne & Mochkovitch 2007) Long –lasting, steady central engine emission? (Troja et al. 2007; Liang et al. 2007a)Late prompt emission? (Ghisellini et al. 2007)

1.LC fitting models:

Smoothed triple power law (STPL)

Smoothed broken power law

Single power law

2. Sample selection:

Criterion: No significant flares in the afterglow phase (reduced Chi2<2).

X-ray sample: 103 bursts from 2 year XRT data (179)

Optical data: from literatures (57 bursts)

5. NormalJet-like decay segments: (1) Data & Sample

Zhang et al. 2006

(Liang et al. 2008, ApJ)

(Liang et al. 2008, ApJ)

(Liang et al. 2008, ApJ)

Detection fraction of the jet break candidates in XRT lightcurves is much lower than that in optical band.

None of the breaks can be graded as “Platinum”!Liang et al. 2008 (see also Judith L. Racusin et al. 2007;

Panaintescu 2007)

5. NormalJet-like decay segments:

(2) Grading jet break candidates

(Progressively rigorous grades)

(Liang et al. 2008, ApJ)

5. NormalJet-like decay segments:(3) Data vs Closure relations (X-rays)

Liang et al. 2008, ApJ (see also Panaitescu 2007, MNRAS)

ISM Wind

Pre-Break

Post-Break

Post-Break

Pre-Break

5. NormalJet-like decay segments:(4) Data vs Closure relations (opt)

Liang et al. 2008, ApJ

ISM Wind

Pre-Break

Pre-Break

Post-Break Post-Break

5. NormalJet-like decay segments:(5) Jet Collimation & Kinetic Energetics

Assuming: ``Silver'' or ``Gold'' jet break candidates are jet breaks+

Following the standard forward shock model tConstraining jet collimation and kinetic energy

Limitsdetection

Liang et al. 2008, ApJ

Pre-Swift

Swift

5. NormalJet-like decay segments:(6)Achromatic vs. chromatic

Liang et al. 2008

13 bursts have good temporal coverage in both X-ray and optical bands:

achromatic : chromatic achromatic : chromatic ==6:76:7

5. NormalJet-like decay segments: (6)Achromatic vs. chromatic

Detection Fraction: (“silver” and “gold”)

X-ray: 27/103 optical: 23/57

Break time and break significance

X-

rays

Op

t.

X-

rays

Op

t.

Liang et al. 2008

X-rays & Opt.: X-rays & Opt.: from the same from the same component?component?

One should be very cautious in claiming a jet break!!

Chromatic breaks!

Challenge to the jet models!

Summary

No significant spectral evolution is observed in the afterglow phase.

The normal decay phase is roughly consistent with external-shock models, with only four significant outliers ( 060413, 060522, 060607A, & 070110).

The properties of the shallow decay segments are also favor the scenario of the refreshed shocks, but a considerable fraction of shallow-to-normal breaks are chromatic, raising an issue of this scenario.

No textbook version of jet break is found in our sample. Candidates to various degrees are identified by relaxing one or more requirements, but the break time is generally chromatic, raising concerns about interpreting the breaks as jet breaks.

Assuming that the candidates are jet breaks, the EK,j distribution is much scattered than the pre-Swift sample 。

Thanks!