The Shallow Decay Segment of GRB X-Ray Afterglow Revisited

Abstract

Based on the early-year observations from Neil Gehrels Swift Observatory, Liang et al. (2007) performed a systematic analysis for the shallow decay component of gamma-ray bursts (GRBs) X-ray afterglow, in order to explore its physical origin. Here we revisit the analysis with an updated sample (with Swift/XRT GRBs between February 2004 and July 2017). We find that with a larger sample, 1) the distributions of the characteristic properties of the shallow decay phase (e.g. t b , S X , Γ X,1 , and α X,1 ) still accords with normal or lognormal distribution; 2) Γ X,1 and Γ γ still show no correlation, but the tentative correlations of durations, energy fluences, and isotropic energies between the gamma-ray and X-ray phases still exist; 3) for most GRBs, there is no significant spectral evolution between the shallow decay segment and its follow-up segment, and the latter is usually consistent with the external-shock models; 4) assuming that the central engine has a power-law luminosity release history as L(t) = L 0 ( t t0 ) −q , we find that the value q is mainly distributed between -0.5 and 0.5, with an average value of 0.16± 0.12; 5) the tentative correlation between E iso,X and t ′ b disappears, so that the global 3-parameter correlation (E iso,X − E ′ p − t ′ b ) becomes less significant; 6) the anti-correlation between L X and t ′ b and the three-parameter correlation (E iso,γ − L X − t b ) indeed exist with a high confidence level. Overall, our results are generally consistent with Liang et al. (2007), confirming their suggestion that the shallow decay segment in most bursts is consistent with an external forward shock origin, probably due to a continuous energy injection from a long-lived central engine.

Less noticeable shallow decay phase in early X-ray afterglows of GeV/TeV-detected gamma-ray bursts

“…Even if X-ray light curve has a break at t b , eight events (GRB 100728A, 110731A, 130427A, 130907A, 140102A, 150314A, 160325A and a VHE event GRB 180720B) have the pre-break decay index α1 larger than 0.7. For the sample of Zhao et al (2019), the distribution of the pre-break decay index has a mean of 0.43 and a dispersion of 0.22 (see dotted line in Fig. 2), hence the 8 events with α1 > 0.7 deviate from the mean value for ordinary GRBs more than 1σ.…”

“…with α1 and α2 describing the decay slopes of pre- and post-break segments, respectively, and t b is a break time. A smoothness parameter w is fixed to be 3 (Liang, Zhang & Zhang 2007;Zhao et al 2019). We compare above two models in order to determine whether the additional degrees of freedom in the DPL model are warranted over a simpler SPL model.…”

“…The initial steep decay phase is most likely the tail emission of the prompt GRB (Kumar & Panaitescu 2000;Zhang, et al 2006;, and the late normal decay phase is well explained by the external forward shock model proposed in the pre-Swift era (Sari, Piran & Narayan 1998). The most enigmatic is the shallow decay phase which typically lasts 10 3−4 s (Willingale, et al 2007;Liang, Zhang & Zhang 2007;Sakamoto, et al 2008;Dainotti, et al 2010Dainotti, et al , 2013Dainotti, et al , 2016Margutti, et al 2013;Tang et al 2019;Zhao et al 2019). Proposed models are the energy injection model (Nousek, et al 2006;Zhang, et al 2006;Granot & Kumar 2006;Kobayashi & Zhang 2007), the inhomogeneous or twocomponent jet model ; Eichler & Granot ⋆ E-mail: ryo@phys.aoyama.ac.jp (RY) 2006; Granot, Königl & Piran 2006;Beniamini et al 2019), the time-dependent microphysics model Granot, Königl & Piran 2006;Fan & Piran 2006), the prior explosion model Yamazaki 2009), the cannonball model (Dado, Dar & De Rújula 2006), the reverse shock-dominated afterglow model (Genet, Daigne & Mochkovitch 2007), the internal engine model (Ghisellini, et al 2007), the super-critical pile model (Sultana, Kazanas & Mastichiadis 2013), the collapsar model ejecting thick shells (van Eerten 2014), and so on.…”

“…Blue and red histograms show the distributions of the decay slope α 1 for 5 SPL and 18 DPL events, respectively. Dashed and dotted lines are those for long GRBs with typical shallow decay phase taken fromTang et al (2019) andZhao et al (2019), respectively. Also shown are allows describing the values of α 1 for 3 VHE events.…”

On the Investigation of the Closure Relations for Gamma-Ray Bursts Observed by Swift in the Post-plateau Phase and the GRB Fundamental Plane

“…The afterglow of GRBs is likely due to the external forward shock (ES) where the relativistic ejecta impacts the external medium (Paczynski & Rhoads 1993;Katz & Piran 1997;Meszaros & Rees 1997). It has already been shown that Swift GRB lightcurves (LCs) have more complex features than a simple power law (PL; Tagliaferri et al 2005;Nousek et al 2006;O'Brien et al 2006;Zhang et al 2006a;Sakamoto et al 2007;Zhao et al 2019). O'Brien et al (2006) and Sakamoto et al (2007) discovered the existence of a flat part in the X-ray LCs of GRBs, called the "plateau," which is present soon after the decaying phase of the prompt emission.…”