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

A plateau phase in the X-ray afterglow is observed in a significant fraction of gamma-ray bursts (GRBs). Previously, a correlation among three key parameters concerning the plateau phase is found to exist, 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 _γ,iso). In this study, we systematically search through all the 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 _γ,iso is confirmed, with the best-fit relation being {L}_{{X}}\propto {T}_a^-1.01{E}_{γ ,{iso}}^0.84. Such an updated three-parameter correlation still supports that the central leftover after GRBs is probably a millisecond magnetar. Note that short GRBs with durations 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 with 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.