Physical Processes Shaping Gamma‐Ray Burst X‐Ray Afterglow Light Curves: Theoretical Implications from the Swift X‐Ray Telescope Observations

“…the external shock (Mészáros & Rees 1993; Dermer & Mitman 1999) or do they come from different locations (i.e., prompt emission from the internal shocks [Rees & Mészáros 1994; Paczynski & Xu 1994], while the afterglow comes from the external shock [Mészáros & Rees 1997a; Sari et al. 1998])?”

“…the external shock (Mészáros & Rees 1993; Dermer & Mitman 1999) or do they come from different locations (i.e., prompt emission from the internal shocks [Rees & Mészáros 1994; Paczynski & Xu 1994], while the afterglow comes from the external shock [Mészáros & Rees 1997a; Sari et al. 1998])?”

“…the external shock (Mészáros & Rees 1993; Dermer & Mitman 1999) or do they come from different locations (i.e., prompt emission from the internal shocks [Rees & Mészáros 1994; Paczynski & Xu 1994], while the afterglow comes from the external shock [Mészáros & Rees 1997a; Sari et al. 1998])?”

“The energy injection is also brief, but the outflow has a significant fraction of Poynting flux (e.g., Usov 1992; Thompson 1994; Mészáros & Rees 1997b; Lyutikov & Blandford 2003).”

“Limited by the quality of the data, the current analyses indicate that the spectral indices remain unchanged in segments II, III, and IV, with a typical value of X 1 (FX / X ; Nousek et al. 2006).”

“Before the Swift era, no solid observation was available to finally differentiate both scenarios, and evidence in favor of each scenario had been collected (for a review see, e.g., Zhang & Mészáros 2004).”

“The jet model (e.g., Rhoads 1999; Sari et al. 1999).”

“This could be because of either a continuous operation of the central engine (Dai & Lu 1998a; Zhang & Mészáros 2001; Dai 2004), or a power-law distribution of the Lorentz factors in the ejecta that results in slower ejecta catching up with the decelerated fireball at later times (Rees&Mészáros…”

“The energy injection is also brief, but the outflow has a significant fraction of Poynting flux (e.g., Usov 1992; Thompson 1994; Mészáros & Rees 1997b; Lyutikov & Blandford 2003).”

“Are prompt emission and afterglow both from the external shock (Mészáros & Rees 1993; Dermer & Mitman 1999) or do they come from different locations (i.e., prompt emission from the internal shocks [Rees & Mészáros 1994; Paczynski & Xu 1994], while the afterglow comes from the external shock…”

“Previous analyses using late-time afterglow data (e.g., Panaitescu & Kumar 2002; Yost et al. 2003) have also suggested that most afterglow data are consistent with an ISMmodel rather than a windmodel.”

“This model has been successful in interpreting the early optical emission from GRB 990123 (Akerlof et al. 1999; Sari & Piran 1999; Mészáros & Rees 1999), GRB 021211 (Fox et al. 2003; Li et al. 2003a; Wei 2003), and GRB 041219a (Blake et al. 2005; Vestrand et al. 2005; Fan et al. 2005c).”

“This is a short, hard burst with T90 3 s whose host galaxy is an elliptical galaxy, similar to the case of the first Swift-localized short burst GRB 050509B (Gehrels et al. 2005).”

“The observed pulses essentially track the behavior of the central engine (Kobayashi et al. 1997).”

“The wind model (e.g., Chevalier & Li 2000).”

“This could potentially explain the X-ray flare (by a factor of 6) detected in GRB 050406 (Burrows et al. 2005a; P. Romano et al. 2006, in preparation).”

“…comes from the internal shocks due to the collisions among many engineejected shells (Rees &Mészáros 1994; Paczynski & Xu 1994) or is due to magnetic or other dissipation processes at a radius smaller than the fireball deceleration radius (e.g., Drenkhahn & Spruit 2002; Rees & Mészáros 2005).”

“The most widely discussed model is the internal shock model (Rees & Mészáros 1994; Paczynski & Xu 1994; Kobayashi et al. 1997; Daigne & Mochkovitch 1998; Fan et al. 2004b).”

“;X contrast is expected. spectrum (e.g., Baring &Harding 1997; Lithwick & Sari 2001), the reverse shock data (Sari & Piran 1999; Wang et al. 2000; Zhang et al. 2003), and the superluminal expansion of the radio afterglow source image (Waxman et al. 1998).”

“The second possibility is that one is looking at a structured jet (Zhang &Mészáros 2002b; Rossi et al. 2002), with the line of sight significantly off-axis.”

“This is the so-called curvature effect (Fenimore et al. 1996; Kumar & Panaitescu 2000b; Dermer 2004; Dyks et al. 2005).”

“…comes from the internal shocks due to the collisions among many engineejected shells (Rees &Mészáros 1994; Paczynski & Xu 1994) or is due to magnetic or other dissipation processes at a radius smaller than the fireball deceleration radius (e.g., Drenkhahn & Spruit 2002; Rees & Mészáros 2005).”

“…the external shock (Mészáros & Rees 1993; Dermer & Mitman 1999) or do they come from different locations (i.e., prompt emission from the internal shocks [Rees & Mészáros 1994; Paczynski & Xu 1994], while the afterglow comes from the external shock [Mészáros & Rees 1997a; Sari et al. 1998])?”

“Flares could also occur if the central engine is a differentially rotating neutron star rather than a black hole (Dai et al. 2006).”

“…by the theoretical argument that the reverse shock emission component plays an important role in the optical band; e.g.,Mészáros& Rees 1997a, 1999; Sari & Piran 1999; Kobayashi 2000; Kobayashi & Zhang 2003a, 2003b; Zhang et al. 2003; Wei 2003; Wu et al. 2003; Li et al. 2003b; Fan et al. 2004a,…”

“…in many GRBs is in great contrast with the intuitive expectations in the external shock model, which generally predicts a smooth burst temporal profile, and it has been argued that the radiative efficiency is too low for the model so that a much larger energy budget is required (Sari & Piran 1997).”

“This gives a physical motivation to the phenomenological two-component jet model (e.g., Lipunov et al. 2001; Berger et al. 2003; Huang et al. 2004).”

“The similar light curves for some long GRBs (e.g., GRB 050502B; Burrows et al. 2005a) and some short GRBs (e.g., GRB 050724; Barthelmy et al. 2005b) indicate that different progenitor systems may share some similarities in the central engine.”

“Such an injection also happens if the central engine further ejects highshells that catch up with the decelerating blast wave (Zhang & Mészáros 2002a).”

“…argument that the reverse shock emission component plays an important role in the optical band; e.g.,Mészáros& Rees 1997a, 1999; Sari & Piran 1999; Kobayashi 2000; Kobayashi & Zhang 2003a, 2003b; Zhang et al. 2003; Wei 2003; Wu et al. 2003; Li et al. 2003b; Fan et al. 2004a, 2005b, 2005c; Zhang…”

“…the external shock (Mészáros & Rees 1993; Dermer & Mitman 1999) or do they come from different locations (i.e., prompt emission from the internal shocks [Rees & Mészáros 1994; Paczynski & Xu 1994], while the afterglow comes from the external shock [Mészáros & Rees 1997a; Sari et al. 1998])?”

“While the early forward shock synchrotron emission peaks in X-rays at early times, the reverse shock synchrotron emission usually peaks in the optical/IR band or even lower (e.g.,Mészáros & Rees 1997a; Sari & Piran 1999; Kobayashi 2000; Zhang et al. 2003; Zhang&Kobayashi 2005).”

“This model has been successful in interpreting the early optical emission from GRB 990123 (Akerlof et al. 1999; Sari & Piran 1999; Mészáros & Rees 1999), GRB 021211 (Fox et al. 2003; Li et al. 2003a; Wei 2003), and GRB 041219a (Blake et al. 2005; Vestrand et al. 2005; Fan et al. 2005c).”

“In the collapsar progenitor models, it is expected that the ejecta generally has two components, one ultrarelativistic component powering the GRB, and another moderately relativistic cocoon component (e.g., Zhang. et al. 2004b; Mészáros & Rees 2001; Ramirez-Ruiz et al. 2002).”

“This is the so-called curvature effect (Fenimore et al. 1996; Kumar & Panaitescu 2000b; Dermer 2004; Dyks et al. 2005).”

“This is an afterglow code developed from the dynamical model of Huang et al. (2000) and has been used in several previous studies (e.g., Fan et al. 2004a, 2005b).”

“…the ejecta has a range of Lorentz factors, e.g., the amount of ejected mass moving with Lorentz factors greater than is (Rees & Mészáros 1998; Panaitescu et al. 1998; Sari & Mészáros 2000) M (> ) / s: ð17Þ 14 As discussed in x 3.2, after collecting more data, we now believe that…”

“In principle, GRB jets could be structured (Zhang & Mészáros 2002b; Rossi et al. 2002; Zhang et al. 2004a; Kumar & Granot 2003).”

“Such an injection also happens if the central engine further ejects highshells that catch up with the decelerating blast wave (Zhang & Mészáros 2002a).”

“(3) Deceleration of a promptly ejected Poynting flux–dominated flow (e.g., Zhang & Kobayashi 2005) naturally gives a smooth injection signature observed.”

“Tagliaferri et al. (2005) explored the t0 effect and concluded that the two distinct light-curve segments are likely intrinsic rather than due to a poor choice of t0.”

“In principle, GRB jets could be structured (Zhang & Mészáros 2002b; Rossi et al. 2002; Zhang et al. 2004a; Kumar & Granot 2003).”

“As a result, asRB 31 andRe 31, the reverse shock synchrotron component would stick out above the forward shock synchrotron component, and an X-ray bump is likely to emerge (see also Fan & Wei 2005).”

“This model has been successful in interpreting the early optical emission from GRB 990123 (Akerlof et al. 1999; Sari & Piran 1999; Mészáros & Rees 1999), GRB 021211 (Fox et al. 2003; Li et al. 2003a; Wei 2003), and GRB 041219a (Blake et al. 2005; Vestrand et al. 2005; Fan et al. 2005c).”

“The light curves for a blast wave surfing on a density wave should be generally quite smooth (cf. Lazzati et al. 2002).”

“;X contrast is expected. spectrum (e.g., Baring &Harding 1997; Lithwick & Sari 2001), the reverse shock data (Sari & Piran 1999; Wang et al. 2000; Zhang et al. 2003), and the superluminal expansion of the radio afterglow source image (Waxman et al. 1998).”

“This could be because of either a continuous operation of the central engine (Dai & Lu 1998a; Zhang & Mészáros 2001; Dai 2004), or a power-law distribution of the Lorentz factors in the ejecta that results in slower ejecta catching up with the decelerated fireball at later times (Rees&Mészáros…”

“A magnetic-dominated accretion flow could also give rise to intermittent accretion flows due to interplay between the gravity and the magnetic barrier (Proga & Begelman 2003; Proga & Zhang 2006).”

“However, in such a case it is very hard to achieve a large contrast between the flares and the underlying afterglow level (Ioka et al. 2005).”

“This is in contrast with the X-ray flare data that show rapid decays right after the flare peak (e.g., Burrows et al. 2005a; Falcone et al. 2006; Romano et al. 2006; O’Brien et al. 2006).”

“In some cases (e.g., GRB 050315; Vaughan et al. 2006), a further steepening is detected after the normal decay phase, which is consistent with a jet break.”

“The star may fragment into several pieces during the collapse (King et al. 2005).”

“Derishev et al. (1999), Beloborodov (2003), and Rossi et al. (2004) suggested that a baryonic fireball contains free neutrons, the decay of whichwould leave important imprints on the light curve.”

“A related model considers a jet with large energy fluctuations in the angular direction, so that its energy distribution is patchy (Kumar & Piran 2000a).”

“This refers to a very high apparent GRB radiation efficiency, even higher than the one estimated using the late X-ray afterglow data (Lloyd-Ronning&Zhang 2004).11 The commonly invoked internal shock model predicts a low-emission efficiency (e.g., Panaitescu et al. 1999; Kumar 1999).”

“…¼ 1:4 ; 104 s 1þ Yð Þ e; 1 B; 2A ¼ 14 sð Þ 1þ Yð Þ e; 1 B; 2A ; 3: ð13Þ The deceleration time is essentially the duration of the burst, i.e., tdec T ¼ (20 s)T1:3, since for typical parameters the wind model is the so-called thick-shell case (e.g., Chevalier & Li 2000; Kobayashi & Zhang 2003b).”

“Dai & Cheng (2001) proposed one scenario to deal with the case of p < 2, while Panaitescu & Kumar (2002) extended the treatment of the p > 2 case to the p 2 regime.”

“This is the so-called curvature effect (Fenimore et al. 1996; Kumar & Panaitescu 2000b; Dermer 2004; Dyks et al. 2005).”

“Another potential test of the curvature effect is to search for a correlation between the spectral peak energy (Epk) and the flux at the peak (Fpk; Dermer 2004).”

“This could be because of either a continuous operation of the central engine (Dai & Lu 1998a; Zhang & Mészáros 2001; Dai 2004), or a power-law distribution of the Lorentz factors in the ejecta that results in slower ejecta catching up with the decelerated fireball at later times (Rees&Mészáros…”

“In order to accommodate the data, it has been suggested that the wind parameter may be small so that at a late enough time the blast wave is already propagating in an ISM (e.g., Chevalier et al. 2004).”

“This model has been successful in interpreting the early optical emission from GRB 990123 (Akerlof et al. 1999; Sari & Piran 1999; Mészáros & Rees 1999), GRB 021211 (Fox et al. 2003; Li et al. 2003a; Wei 2003), and GRB 041219a (Blake et al. 2005; Vestrand et al. 2005; Fan et al. 2005c).”

“This gives a physical motivation to the phenomenological two-component jet model (e.g., Lipunov et al. 2001; Berger et al. 2003; Huang et al. 2004).”

“The latest addition is to also include the kinetic evolution of the electron distribution (Moderski et al. 2000).”

“Understanding such a high apparent radiation efficiency is therefore desirable (see, e.g., Beloborodov 2000; Kobayashi & Sari 2001).”

“A similar feature was evident for GRB 011121 detected by BeppoSAX (Piro et al. 2005).”

“This refers to a very high apparent GRB radiation efficiency, even higher than the one estimated using the late X-ray afterglow data (Lloyd-Ronning&Zhang 2004).11 The commonly invoked internal shock model predicts a low-emission efficiency (e.g., Panaitescu et al. 1999; Kumar 1999).”

“…¼ 1:4 ; 104 s 1þ Yð Þ e; 1 B; 2A ¼ 14 sð Þ 1þ Yð Þ e; 1 B; 2A ; 3: ð13Þ The deceleration time is essentially the duration of the burst, i.e., tdec T ¼ (20 s)T1:3, since for typical parameters the wind model is the so-called thick-shell case (e.g., Chevalier & Li 2000; Kobayashi & Zhang 2003b).”

“…e.g.,Mészáros& Rees 1997a, 1999; Sari & Piran 1999; Kobayashi 2000; Kobayashi & Zhang 2003a, 2003b; Zhang et al. 2003; Wei 2003; Wu et al. 2003; Li et al. 2003b; Fan et al. 2004a, 2005b, 2005c; Zhang & Kobayashi 2005; Nakar & Piran 2004;McMahon et al. 2004), possible early X-ray afterglow…”

“When becomes smaller than 1j , the light curve steepens because of the combination of the jet edge effect and the possible sideways expansion effect (Rhoads 1999; Sari et al. 1999; Panaitescu & Mészáros 1999).”

“…Kobayashi & Zhang 2003a, 2003b; Zhang et al. 2003; Wei 2003; Wu et al. 2003; Li et al. 2003b; Fan et al. 2004a, 2005b, 2005c; Zhang & Kobayashi 2005; Nakar & Piran 2004;McMahon et al. 2004), possible early X-ray afterglow signatures have been only sparsely studied (e.g., Kumar& Panaitescu 2000b;…”

“It has been found that RB is indeed larger than unity for GRB 990123 and GRB 021211 (Zhang et al. 2003; Fan et al. 2002; Kumar & Panaitescu 2003; Panaitescu & Kumar 2004; McMahon et al. 2004).”

“This model has been successful in interpreting the early optical emission from GRB 990123 (Akerlof et al. 1999; Sari & Piran 1999; Mészáros & Rees 1999), GRB 021211 (Fox et al. 2003; Li et al. 2003a; Wei 2003), and GRB 041219a (Blake et al. 2005; Vestrand et al. 2005; Fan et al. 2005c).”

“…the optical band; e.g.,Mészáros& Rees 1997a, 1999; Sari & Piran 1999; Kobayashi 2000; Kobayashi & Zhang 2003a, 2003b; Zhang et al. 2003; Wei 2003; Wu et al. 2003; Li et al. 2003b; Fan et al. 2004a, 2005b, 2005c; Zhang & Kobayashi 2005; Nakar & Piran 2004;McMahon et al. 2004), possible early…”

“This conclusion also applies to other well-studied Swift bursts (e.g., GRB 050525a; Blustin et al. 2006).”

“The similar light curves for some long GRBs (e.g., GRB 050502B; Burrows et al. 2005a) and some short GRBs (e.g., GRB 050724; Barthelmy et al. 2005b) indicate that different progenitor systems may share some similarities in the central engine.”

“Are prompt emission and afterglow both from the external shock (Mészáros & Rees 1993; Dermer & Mitman 1999) or do they come from different locations (i.e., prompt emission from the internal shocks [Rees & Mészáros 1994; Paczynski & Xu 1994], while the afterglow comes from the external shock…”

“Fan et al. (2002) performed a detailed fit to the GRB 990123 data and obtained re ¼ 4:7 fe and rB ¼ 400 f B .”

“This would correspond to a patchy shell (Kumar & Piran 2000b) or a minijet (Yamazaki et al. 2004).”

“Understanding such a high apparent radiation efficiency is therefore desirable (see, e.g., Beloborodov 2000; Kobayashi & Sari 2001).”

“…Rees 1997a, 1999; Sari & Piran 1999; Kobayashi 2000; Kobayashi & Zhang 2003a, 2003b; Zhang et al. 2003; Wei 2003; Wu et al. 2003; Li et al. 2003b; Fan et al. 2004a, 2005b, 2005c; Zhang & Kobayashi 2005; Nakar & Piran 2004;McMahon et al. 2004), possible early X-ray afterglow signatures have…”

“This gives a physical motivation to the phenomenological two-component jet model (e.g., Lipunov et al. 2001; Berger et al. 2003; Huang et al. 2004).”

“=3 at later times (MacFadyen et al. 2001; Janiuk et al. 2004), which has no effect on the blast wave evolution.”

“The optical light curves of the two-component jetmodel have been calculated byHuang et al. (2004) and Granot (2005).”

“It has been found that RB is indeed larger than unity for GRB 990123 and GRB 021211 (Zhang et al. 2003; Fan et al. 2002; Kumar & Panaitescu 2003; Panaitescu & Kumar 2004; McMahon et al. 2004).”

“The interaction of a fireball with density bumps has been studied previously by many authors (e.g., Lazzati et al. 2002; Dai & Wu 2003).”

“Our detailed calculations suggest that the variation caused by density inhomogeneities is generally not very significant (see also Ramirez-Ruiz et al. 2005).”

“Derishev et al. (1999), Beloborodov (2003), and Rossi et al. (2004) suggested that a baryonic fireball contains free neutrons, the decay of whichwould leave important imprints on the light curve.”

“Fan et al. (2005a) argued that at least for the flares following short GRBs, the process that powers the flares has to be of magnetic origin.”

“GRB 050319 (Cusumano et al. 2006).”

“No sideways expansion of the jet is included, which is consistent with previous simulations (Kumar & Granot 2003; Cannizzo et al. 2004).”

“All kinematic effects that affect the observed flux (e.g., Doppler boost, propagation time effects) have been taken into account rigorously, following Salmonson (2003).”

“;X contrast is expected. spectrum (e.g., Baring &Harding 1997; Lithwick & Sari 2001), the reverse shock data (Sari & Piran 1999; Wang et al. 2000; Zhang et al. 2003), and the superluminal expansion of the radio afterglow source image (Waxman et al. 1998).”

“…Rees 1997a, 1999; Sari & Piran 1999; Kobayashi 2000; Kobayashi & Zhang 2003a, 2003b; Zhang et al. 2003; Wei 2003; Wu et al. 2003; Li et al. 2003b; Fan et al. 2004a, 2005b, 2005c; Zhang & Kobayashi 2005; Nakar & Piran 2004;McMahon et al. 2004), possible early X-ray afterglow signatures have…”

“…role in the optical band; e.g.,Mészáros& Rees 1997a, 1999; Sari & Piran 1999; Kobayashi 2000; Kobayashi & Zhang 2003a, 2003b; Zhang et al. 2003; Wei 2003; Wu et al. 2003; Li et al. 2003b; Fan et al. 2004a, 2005b, 2005c; Zhang & Kobayashi 2005; Nakar & Piran 2004;McMahon et al. 2004),…”

“…is that c increases 12 In certain parameter regimes, the condition tdec < tmc could be satisfied, and in the temporal regime t < tmc, the blast wave is in the fast cooling ( c < m) domain, so the radiative loss could affect the blast wave dynamics (e.g., Böttcher & Dermer 2000; Wu et al. 2005).”

“…is that c increases 12 In certain parameter regimes, the condition tdec < tmc could be satisfied, and in the temporal regime t < tmc, the blast wave is in the fast cooling ( c < m) domain, so the radiative loss could affect the blast wave dynamics (e.g., Böttcher & Dermer 2000; Wu et al. 2005).”

“For the latter, the timescale for the light curve to achieve the asymptotic p index is typically longer than that in the ISM case (e.g., Kumar & Panaitescu 2000a; Gou et al. 2001).”

“…2003a, 2003b; Zhang et al. 2003; Wei 2003; Wu et al. 2003; Li et al. 2003b; Fan et al. 2004a, 2005b, 2005c; Zhang & Kobayashi 2005; Nakar & Piran 2004;McMahon et al. 2004), possible early X-ray afterglow signatures have been only sparsely studied (e.g., Kumar& Panaitescu 2000b; Kobayashi et al.…”

“The injection signature is also inferred in other bursts such as GRB 050117 (Hill et al. 2006) and XRF 050416 (Sakamoto et al. 2006), where similar conclusions could be drawn.”

“The UVOT observations are also consistent with such a picture (Mason et al. 2006).”

“…e.g.,Mészáros& Rees 1997a, 1999; Sari & Piran 1999; Kobayashi 2000; Kobayashi & Zhang 2003a, 2003b; Zhang et al. 2003; Wei 2003; Wu et al. 2003; Li et al. 2003b; Fan et al. 2004a, 2005b, 2005c; Zhang & Kobayashi 2005; Nakar & Piran 2004;McMahon et al. 2004), possible early X-ray afterglow…”

“Fan et al. (2005a) argued that at least for the flares following short GRBs, the process that powers the flares has to be of magnetic origin.”

“In principle, the first scenario may give rise to additional observational signatures (e.g., Rees & Mészáros 2000; Gao & Wei 2004, 2005), which may be used to differentiate the model from the others.”

“In principle, the first scenario may give rise to additional observational signatures (e.g., Rees & Mészáros 2000; Gao & Wei 2004, 2005), which may be used to differentiate the model from the others.”

“There are many projects that LSST data products will enable: • Detection and measurement of gamma-ray burst afterglows and transients (e.g., Zhang & Mészáros 2004; Zhang et al. 2006; Kann et al. 2010) to high redshift (∼7.5).”

“…simply the high latitude emission (described in §2.1) associated with the prompt γ-ray source at R >∼ 1015cm when the central engine turns off faster than the decline of the X-ray lightcurve (Kumar and Panaitescu, 2000a; Dermer, 2004; Zhang et al., 2006; Nousek et al., 2006; Liang et al., 2006a).”