The shallow decay segment of GRB X-ray afterglow revisited

Abstract

Based on the early-year observations from Neil Gehrels \emph{Swift} Observatory, \cite{liang07} 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. $t_b$ , $S_X$, $\hat{I}{“}_{X,1}$, and $\hat{I}{\pm }_{X,1}$) still accords with normal or lognormal distribution; 2) $\hat{I}{“}_{X,1}$ and $\hat{I}{“}_{\hat{I}}^3$ 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 $L(t)=L_0(\frac{t}{t_0}{)}^{-q}$, we find that the value $q$ is mainly distributed between -0.5 and 0.5, with an average value of 0.16$\pm$ 0.12; 5) the tentative correlation between $E_{\mathrm{iso},\mathrm{X}}$ and $t_b^{\prime }$ disappears, so that the global 3-parameter correlation ($E_{\mathrm{iso},\mathrm{X}}-E_p^{\prime }-t_b^{\prime }$) becomes less significant; 6) the anti-correlation between $L_X$ and $t_b^{\prime }$ and the three-parameter correlation ($E_{\mathrm{iso},{\hat{\mathrm{I}}}^3}-L_X-t_b$) indeed exist with a high confidence level. Overall, our results are generally consistent with \cite{liang07}, 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.

accepted for publication by ApJ