New Equations of State for Postmerger Supramassive Quark Stars

Abstract

Abstract Binary neutron star (NS) mergers with their subsequent fast-rotating supramassive magnetars are one attractive interpretation for at least some short gamma-ray bursts (SGRBs), based on the internal plateau commonly observed in the early X-ray afterglow. The rapid decay phase in this scenario signifies the epoch when the star collapses to a black hole after it spins down, and could effectively shed light on the underlying unclear equation of state (EoS) of dense matter. In the present work, we compare the protomagnetar masses of the internal plateau sample from representative EoS models to that derived independently from the observed galactic NS–NS binary, aiming to contribute new compact star EoSs from SGRB observations. For this purpose, we employ various EoSs covering a wide range of maximum masses for both NSs and quark stars (QSs) and, at the same time, satisfying the recent observational constraints of the two massive pulsars for which the masses are precisely measured (around ). We first illustrate that how well the underlying EoS would reconcile with the current posterior mass distribution is largely determined by the static maximum mass of that EoS. We then construct three new postmerger QS EoSs (PMQS1, PMQS2, PMQS3), fully respecting the observed distribution. We also provide easy-to-use parameterizations for both the EoSs and the corresponding maximum gravitational masses of rotating stars. In addition, we calculate the fractions of postmerger products for each EoS, and discuss potential consequences for the magnetar-powered kilonova model.