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. 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 2004 February and 2017 July). We find that with a larger sample, (1) the distributions of the characteristic properties of the shallow decay phase (e.g., t <inf>b</inf>, S <inf>X</inf>, Γ<inf>X,1</inf>, and α <inf>X,1</inf>) still accord with normal or lognormal distribution; (2) Γ<inf>X,1</inf> and Γ<inf>γ</inf> 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 , 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 and disappears, so that the global three-parameter correlation () becomes less significant; (6) the anticorrelation between L <inf>X</inf> and and the three-parameter correlation () indeed exist with a high confidence level. Overall, our results are generally consistent with Liang et al., 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.