Generating Initial Data and Evolving Hypermassive Neutron Stars

Binary neutron star(BNS) mergers are ideal laboratories for studying matter under extreme conditions. The post-merger remnant can persist for many dynamical timescales in quasi-equilibrium configurations or collapse into a black hole, the presence and timescale of which depend crucially on the stability of these equilibria. The turning point criterion is a crucial tool for inferring the stability of NS from its equilibrium sequences. We study the applicability of the criterion for realistic rotation and a range of entropy profiles resembling a post-merger neutron star. The dynamics of the post-merger remnant depends on the momentum transport mechanisms, which could be caused by magnetohydrodynamic instabilities. These instabilities could be modeled using effective viscosity since both viscosity and magnetic fields change the angular velocity of a differentially rotating star on a secular timescale. We present and compare different viscosity models and provide an update on post-merger neutron star simulations with Spectral Einstein Code.