Statistical Study of Gamma-Ray Bursts with a Plateau Phase in the X-ray Afterglow

Abstract

A plateau phase in the X-ray afterglow is observed in a significant fraction of gamma-ray bursts (GRBs). Previously, it has been found that there exists a correlation among three key parameters concerning the plateau phase, i.e., the end time of the plateau phase in the GRB rest frame ($T_a$), the corresponding X-ray luminosity at the end time ($L_X$) and the isotropic energy of the prompt GRB ($E_{\gamma ,\mathrm{iso}}$). In this study, we systematically search through all the \emph{Swift} GRBs with a plateau phase that occurred between 2005 May and 2018 August. We collect 174 GRBs, with redshifts available for all of them. For the whole sample, the correlation between $L_X$, $T_a$ and $E_{\gamma ,\mathrm{iso}}$ is confirmed, with the best fit relation being $L_X\propto T_a^{-1.01}{E}_{\gamma ,\mathrm{iso}}^{0.84}$. Such an updated three-parameter correlation still supports that the central leftover after GRBs is probably a millisecond magnetar. It is interesting to note that short GRBs with duration less than 2 s in our sample also follow the same correlation, which hints that the merger production of two neutron stars could be a high mass magnetar, but not necessarily a black hole. Moreover, GRBs having an "internal" plateau (i.e., with a following decay index being generally smaller than -3) also obey this correlation. It further strengthens the idea that the internal plateau is due to the delayed collapse of a high mass neutron star into a black hole. The updated three-parameter correlation indicates that GRBs with a plateau phase may act as a standard candle for cosmology study.

35 pages, 8 figures, 3 tables, accepted by AAS Journal