Gamma-Ray Burst Afterglow

Transcription

1 Gamma-Ray Burst Afterglow Bing Zhang Department of Physics and Astronomy University of Nevada Las Vegas May 29, 2009, KIAA-PKU

2 Lecture series GRB overview Very general overview of the GRB field to general audience - maybe still somewhat informative for GRB researchers GRB afterglow Basic external shock afterglow theory, and new afterglow concept in the Swift era GRB prompt emission Difficult topic: composition (what), dissipation radius (where), dissipation mechanism, radiation mechanism (how) GRB diversity & classification Short vs.. Long; Type I vs.. II; HL vs.. LL

3 Generic Fireball Shock Model (Paczynski, Meszaros,, Rees, Sari, Piran, )

4 Two frames, three times Zhang & Meszaros (2004) Propagation effect Lorentz transformation Combination of the two effects

5 Fireball Evolution Log Γ Γ R Γ = const. Γ R -3/2 Thin shell R c R IS R dec Log R R ph Meszaros,, Laguna & Rees 93 Piran, Shemi & Narayan 93 Kobayashi, Piran & Sari 99 Thick shell

6 Internal shock tree plot First shell evolution Maxham & Zhang 2009

7 Deceleration Dynamics Energy conservation: E = Γ 0 M 0 c 2 = Γ [M 0 + (Γ( - 1) m] c 2 = const. (Γ - 1) m << M 0 : Γ 0 ~ Γ: : coasting, not decelerated (Γ - 1) m >> M 0 Deceleration regime: E ~ Γ 2 m c 2 ~ Γ 2 ρ c 2 R 3 ~ const. So: Γ R -3/2 if ρ ~ const. (ISM); Γ R -1/2 if ρ R -2 (wind) Deceleration radius: (Γ - 1) m ~ M 0 or m ~ M 0 / Γ

8 Time Dependences ISM: Γ R -3/2 R ~ c t, so Γ t -3/2 t ~ 2 Γ 2 t Γ t -3/8, and R t 1/4 Wind: Γ R -1/2 R ~ c t, so Γ t -1/2 t ~ 2 Γ 2 t Γ t -1/2, and R t 1/2 As a function of the observed time, the dependences become shallower, because the relativistic correction becomes progressively weaker

9 Relativistic shocks Un-shocked ejecta Shocked ejecta Shocked medium Un-shocked medium RS (reverse shock) CD (contact discontinuity) FS (forward shock) n 2 = (4 γ ) n 1 e 2 = (γ( 21-1) n 2 m p c 2 n 3 = (4 γ ) n 4 e 3 = (γ( 34-1) n 3 m p c 2 e 2 = e 3 γ 2 = γ 3 γ 34 = (γ 3 / γ 4 + γ 4 / γ 3 ) / 2

10 Equipartition parameters e 2 = e p,2 + e e,2 + e B,2 = (ε( p + ε e + ε B ) e 2 ε p + ε e + ε B = 1 The shock internal energy is postulated to be distributed among ions (protons), electrons and magnetic fields non-evenly. Their values are constrained through afterglow modeling

11 Synchrotron radiation (1) Single electron emission; Emission from power- law electrons: N(γ) γ Cooling spectrum; Meszaros & Rees 1997; Sari, Piran & Narayan 1998 γ -p

12 Synchrotron radiation (2) Continuous acceleration; Self-absorption Slow cooling Fast cooling Meszaros & Rees 1997; Sari, Piran & Narayan 1998

13 ISM afterglow model Sari, Piran & Narayan (1998)

14 Wind afterglow model Chevalier & Li (2000)

15 GRB collimation (Jet) Rhoads 1997, 1999; Sari et al Structured vs.. uniform jets Zhang & Meszaros 2002 Rossi, Lazzati & Rees 2002

16 Afterglow Closure Relations Sari, Piran & Narayan (1998) Chevalier & Li (2000) Dai & Cheng (2001) Zhang & Meszaros (2004)

17 Confronting data with theory Wijers & Galama 99 Stanek et al. 99

18 Confronting data with theory Panaitescu & Kumar (2001)

19 Forward vs.. reverse shocks Un-shocked ejecta Shocked ejecta Shocked medium Un-shocked medium RS (reverse shock) CD (contact discontinuity) FS (forward shock) γ 2 = γ 3 = γ m 2 ~ m 3 / γ (deceleration condition) n 2 ~ n 3 / γ (shocked shell is denser than shocked medium) e 2 = e 3 γ e,2 ~ γ γ e,3 (shell electrons are less energetic than medium electrons) FS: X-ray emission; RS: optical flash

20 FS - RS relation at shock crossing time Zhang, Kobayashi & Meszaros (2003)

21 FS - RS optical lightcurves Zhang, Kobayashi & Meszaros (2003)

22 GRB (Akerlof et al. 1999) Meszaros & Rees 1997, 1999 Sari & Piran 1999a, 1999b R B = B r / B f ~ 15 Zhang, Kobayashi & Meszaros, 2003 Fan et al. 2002

23 GRB (Fox et al. 2003; Li et al. 2003) R B = B r / B f >> 1 Zhang, Kobayashi & Meszaros, 2003 Kumar & Panaitescu 2003

24 GRB A (Vestrand et al. 2005; Blake et al. 2005) R B = B r / B f ~ 3 Fan, Zhang & Wei 2005

25 An analytic MHD shock Solution for GRB reverse shocks (Zhang & Kobayashi 2004) Two free parameters: σ, γ 34 σ = 0 Blandford-McKee (1976) γ 34 = Kennel-Coroniti (1984)

26 t 1/2 t -1 Optical, forward shock emission

27 t? t -2 t 1/2 t -1 Optical, forward + reverse shock emission σ << 1

28 t? t -2 t 1/2 t -1 Optical, forward + reverse shock emission σ ~ 0.01

29 t -2 t? t 1/2 t -1 Optical, forward + reverse shock emission σ ~ 1

30 t 1/2 t -1 Optical, forward + reverse shock emission σ ~ 10

31 t 1/2 t -1 Optical, forward + reverse shock emission σ ~ 100

32 A third regime for σ << 1 GRB Molinari et al. (2007) Jin & Fan (2007)

33 Refreshed shocks Zhang & Meszaros (2002) Rees & Meszaros (1998); Sari & Meszaros (2001); Kumar & Piran (2000)

34 Early XRT afterglow (Nousek et al. 2006; O Brien et al., 2006)

35 A Five-Component Canonical X-Ray Afterglow ~ -3 I V Zhang et al. (2006) II ~ -0.5 III 10^4 10^5 s ~ ^2 10^3 s 10^3 10^4 s IV ~ -2

36 Five components A steep decay - GRB tail emission A shallow-than-normal decay - refreshed shock A normal decay - a fireball with constant energy running into a medium with a constant density A possible jet break One or more X-ray flares Although the 3rd and 4th components are expected, the other three components are surprises to GRB workers.

37 Canonical lightcurves: Internal or external? (Zhang et al. 2006; Nousek et al. 2006) Curvature tail Internal emission I V Late central engine activity Continuous energy injection II III External forward shock emission? Normal decay Post jet break decay IV

38 Steep decay interpretation Tail of prompt GRB emission curvature effect (Kumar & Panaitescu 2000; Zhang et al. 2006; Dyks et al. 2006) Important implication: GRBs and afterglows come from different locations! -α -β α = β + 2, F = t ν tail GRB afterglow But see: Pe er er et al Dermer 2008 Duran & Kumar 2009

39 Spectral Evolution & Non-Power-Law Curvature Effect B.-B. Zhang, Liang & Zhang (2007) B.-B. Zhang et al. (2009)

40 X-ray flare interpretation: Late central engine activity (Burrows et al 2005; Zhang et al. 2006; Fan & Wei 2005; Wu et al. 2006; Liang et al. 2006; Lazzati & Perna 2007; Chincarini et al. 2007; Falcone et al. 2007; Maxham & Zhang 2007) Can naturally interpret rapid rise and rapid fall of the lightcurves. A much smaller energy budget is needed. central photosphere internal external shocks engine (shocks) (reverse) (forward)

41 Clue: again rapid decay Rapid decays are following both prompt emission and X- ray flares Very likely it is due to high-latitude emission upon sudden cessation of emission curvature effect (Kumar & Panaitescu 2000; Dermer 2004; Zhang et al. 2006; Fan & Wei 2005; Panaitescu et al. 2006; Dyks et al. 2006) -α -β α = β + 2, F = t ν ν tail GRB afterglow

42 Complications (1): T0 Zhang et al. (2005)

43 Complications (2): superposition GRB & flare tail emission Observed Underlying forward shock emission Zhang et al. (2005)

44 Testing curvature effect interpretation Liang et al. (2006) Assume the rapid decay is the superposition of tail emission ( (α = β + 2) ) and the underlying forward shock emission Search for T0. Is s T0 consistent with the expectation, i.e. the beginning of the flare or last pulse in the prompt emission?

45 Testing curvature effect interpretation Liang et al. (2005)

46 Testing curvature effect interpretation Liang et al. (2005) The long GRB B and the short GRB have similar observational properties!

47 X-ray flare origin The rapid decay following X-ray flares is consistent with the curvature effect interpretation X-ray flares therefore are of internal origin, caused by late central engine activity

48 Late internal shock model requirements Injection must be intermittent - late injection produces late flares Late injection episodes are less energetic Late injection has longer durations in order to produce fatter flares at late times Maxham & Zhang (2009)

49 X-Ray flare Idea 1: Fragmentation of the Star (King et al. 2005) stellar core

50 X-Ray Flare Idea 2: Fragmentation of the Disk (Perna, Armitage & Zhang, 2006) B H

51 X-Ray Flare Idea 3: Magnetic barrier modulated accretion flow (Proga & Zhang, 2006)

52 X-Ray Flare Idea 4: Post-merger millisecond pulsar (Dai et al, 2006) An alternative idea: He-synthesis in the disk (Lee et al. 2009)

53 Shallow decay (plateau) interpretations Continuous energy injection into the forward shock (Zhang et al. 2006; Nousek et al. 2006; Panaitescu et al. 2006) Long term central engine (Dai & Lu 1998; Zhang & Meszaros 2001) A distribution of Lorentz factor (Rees & Meszaros 1998; Sari & Meszaros 2001) Evolution microscopic parameters (Fan & Piran 2006; Ioka et al. 2006) Long-term reverse shock (Genet et al. 2007; Uhm & Beloborodov 2007) Dust scattering (Shao( & Dai 2007; Shen et al. 2008) Scattering of FS photons by a late lepton-rich ejecta (Panaitescu 2008) Central engine afterglow (Ghisellini( et al. 2007; Kumar et al. 2008) Two component jets (De Pasquale et al. 2008) Prior emission (Ioka( et al. 2006; Yamazaki 2009; Liang et al. 2009)

54 Puzzling fact: Chromatic breaks I V Panaitescu et al Fan & Piran 2006 Huang et al Urata et al Liang et al Chandra et al II Optical light curve III IV

55 Chromatic vs.. achromatic breaks (Liang,, Zhang & Zhang 2007) Chromatic break at end of internal X-ray plateaus (2/13): most natural Chromatic break at end of normal X-ray plateaus (5/13): most puzzling Achromatic break at end of normal X-ray plateaus (6/13): consistent with forward shock origin

56 Consistency of X-rays with the forward shock models (Willingale et al. 2007; Liang et al. 2007; Panaitescu 2007) X-rays are easier (than optical) to be confronted with the forward shock models (both α X and β X can be measured) - more difficult to measure β opt In the normal decay phase (III), X-rays are generally consistent with the forward shock models.

57 New additions: internal plateaus I II V VI? III GRB Troja et al Liang et al /53 plateaus, a small fraction! IV Many X-ray flares? Spindown of a central pulsar-like object?

58 (Panaitescu 2008; Butler & Jet breaks? (Liang et al. 2008) 2008; Butler & Kocevski 2008; Curan et al. 2008; Racusin et al. 2009; Cenko et al. 2009) Achromatic break Break in one band Chromatic breaks One should to be cautious to claim collimation & energetics!

59 Conclusions The external forward shock afterglow model is a generic model. Some GRBs can be well interpreted by this model. Swift revolutionized our view on GRBs: : The observed afterglow emission is the superposition of internal and external emission components. At least X-ray flares and a few internal plateaus are of internal origin, which demand late central engine activity. The nature of the shallow decay (plateau) component (followed by a normal decay) is still a mystery. The origin of afterglow temporal breaks (especially those chromatic ones) is unknown. One needs to be cautious to interpret some observed breaks as jet breaks. In more than half GRBs,, X-rays and optical likely come from different emission components.