New Equations of State for Postmerger Supramassive Quark Stars

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 confront the protomagnetar masses of the internal plateau sample from representative EoS models, with the one 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 mass for both NSs and quark stars (QSs), and in the same time satisfying the recent observational constraints of the two massive pulsars whose masses are precisely measured (around 2M ⊙ ). 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 3 new postmerger QS EoSs (PMQS1, PMQS2, PMQS3), respecting fully 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.

Internal Plateau in Short GRBs and Quark Stars

“…See texts for details. [3] are collected in Table I. For a given EoS, the rns code (http : //www.gravity.phys.uwm.edu/rns/) presents uniformly rotating, axisymmetric, equatorially symmetric configurations of a NS/QS.…”

“…The corresponding increase in the corresponding equatorial radius R eq is 31 − 36% (57 − 60%). For later use, M max can be fitted well as a function of the spin period (P ) [2,3]: M max = M TOV (1 + αP β ). The fitting parameters (α, β) are also shown in Table I.…”

“…[3]. We also do the rns calculates for the new EoSs and fit their fast-rotating configurations in analytic forms.…”

“…[3] for PMQS1, PMQS2, and PMQS3. The pressure is calculated through the standard formulae to fulfill the thermodynamical equilibrium: P = n 2 b (∂(ε/n b )/∂n b ).…”

“…More details can be found in Refs. [2,3] and references therein. In the QS central engine model (Fig.…”