Global Properties of X-ray Afterglows Observed with XRT ENWEI LIANG （梁恩维） University of Guangxi, Nanning astro.gxu.edu.cn Nanjing

Collaborators BING ZHANG (UNLV) BING ZHANG (UNLV) BINBIN ZHANG (UNLV) BINBIN ZHANG (UNLV) Judith L. Racusin PENN. STATE UNIVERSITY Judith L. Racusin (PENN. STATE UNIVERSITY) DAVE BURROWS (PENN. STATE UNIVERSITY) DAVE BURROWS (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 Canonical XRT LC X-ray Flares Non-Canonical XRT LC Lv et al. 2008( in preparation) BinBin’s talkThis 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 correction –Extract the BAT and XRT lightcurves at any user-specified temporal and spectral regimes –Automatically extract the BAT and XRT spectra and fit with Xspec –Output eps figures for publication Scripts and data are available at : – UNLV) – GXU)

3.What we did: A comprehensive analysis Data: two-year XRT observations. Temporal analysis Spectral analysis Data vs Fireball model: Challenges to conventional GRB models

 LC fitting model: Broken power law  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.  Sample: 52 bursts from 2 year XRT data (179) 4. Shallow  Normal decay segments: (1)Data & Analysis method Zhang et al (Liang et al. 2007, ApJ)

4. Shallow  Normal decay segments: (2)Distributions of Characteristcs (Liang et al. 2007, ApJ) Break timeX-ray fluence Ph. Index Decay slope

4. Shallow  Normal decay segments: (3) Correlations with Prompt Emisison (Liang et al. 2007, ApJ) Duration Ph. Index fluence Energy

4. Shallow  Normal decay segments: (4)spectral evolution! (Liang et al. 2007, ApJ)

4. Shallow  Normal decay segments: (5) Data vs closure relations  Most of the normal decay segments are roughly consistent with the closure relations  Energy Injection?  Four cases are not  Internal origin?. (Liang et al. 2007, ApJ)

4. Shallow  Normal decay segments: (6) Energy injection? Liang-Zhang relation (Liang et al. 2007) E iso ∝ t b,opt -1.2

4. Shallow  Normal decay segments: (6) X-Ray vs Optical (a) (Liang et al. 2007) Achromatic!

4. Shallow  Normal 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)

 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 Chi 2 <2).  X-ray sample: 103 bursts from 2 year XRT data (179)  Optical data: from literatures (57 bursts) 5. Normal  Jet-like decay segments: (1) Data & Sample Zhang et al (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 (see also Judith L. Racusin et al. 2007; Panaintescu 2007) 5. Normal  Jet-like decay segments: (2) Grading jet break candidates (Progressively rigorous grades)

(Liang et al. 2008, ApJ)

5. Normal  Jet-like decay segments: (3) Data vs Closure relations (X-rays) Liang et al. 2008, ApJ (see also Panaitescu 2007, MNRAS) ISMWind Pre-Break Post-Break Pre-Break

5. Normal  Jet-like decay segments: (4) Data vs Closure relations (opt) Liang et al. 2008, ApJ ISMWind Pre-Break Post-Break

5. Normal  Jet-like decay segments: (5) Jet Collimation & Kinetic Energetics Assuming: ``Silver'' or ``Gold'' jet break candidates are jet breaks+ Following the standard forward shock model t  Constraining jet collimation and kinetic energy Limits dete c t i o n Liang et al. 2008, ApJ Pre-Swift Swift

5. Normal  Jet-like decay segments: (6)Achromatic vs. chromatic Liang et al bursts have good temporal coverage in both X-ray and optical bands: achromatic : chromatic =6:7

5. Normal  Jet-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 Opt. X-rays Opt. Liang et al. 2008

X-rays & Opt.: from the same 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 ( , , A, & ). 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 E K,j distribution is much scattered than the pre-Swift sample 。

Thanks!