Abstract
Numerical simulations of binary neutron star mergers invariably show that, when a long-lived remnant forms, its rotation profile is never a simple decaying function of the radius but rather exhibits a maximum rotation rate shifted away from the center. This is in contrast to the usual differential rotation profile employed for the numerical modeling of axisymmetric equilibria of relativistic stars. Two families of rotation rate functions that mimic post-merger profiles were proposed by Uryū et al. (2017). In this work we implement Uryū’s profiles into the XNS code by Bucciantini and Del Zanna (2011) and we present novel equilibrium sequences of differentially rotating neutron stars. These are constructed by using three different equations of state, in order to study the dependence of mass, radius, angular momentum, and other important physical quantities, especially the quadrupole deformation and metric quadrupole moment, from the rotation properties.
Highlights
On August 2017, the Advanced Virgo detector and the two Advanced LIGO detectors coherently observed a transient gravitational-wave (GW) signal (GW170817) produced by the coalescence of a binary neutron star (BNS) system [1]
When a long-lived remnant forms from a BNS merger, these numerical simulations show that the resulting hypermassive neutron star (HMNS) is supported against collapse toward a black hole by strong differential rotation gradients
The remnant typically presents a rotation profile which is shallow at the center, followed by a rapid increase toward a maximum, located at a few kilometers away from the center, beyond which the rotation rate drops following an almost Keplerian trend
Summary
On August 2017, the Advanced Virgo detector and the two Advanced LIGO detectors coherently observed a transient gravitational-wave (GW) signal (GW170817) produced by the coalescence of a binary neutron star (BNS) system [1]. In order to investigate the physics of BNS remnants and HMNSs, for instance the production of GWs or their further evolution, it is useful to extend the numerical tools to build equilibria of relativistic stars, usually assuming rigid rotation or simple monotonic profiles (e.g., [18–20]), by allowing for these remnant-like differential rotation profiles. In the present work we investigate in detail the properties of BNS merger remnants, here modelled as axisymmetric, stationary, differentially rotating neutron stars, using different families of barotropic (zero temperature) equations of state (EoSs), and assuming the rotation profiles by Uryu. The XNS code allows to compute magnetized equilibrium models with poloidal, toroidal and mixed magnetic fields and the implementation of post-merger-like rotational profiles is useful in view of future studies of GRMHD dynamics using the ECHO code [33] or similar.
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