Beyond second-order convergence in simulations of magnetized binary neutron stars with realistic microphysics

Abstract

We investigate the impact of using high-order numerical methods to study the merger of magnetised neutron stars with finite-temperature microphysics and neutrino cooling in full general relativity. By implementing a fourth-order accurate conservative finite-difference scheme we model the inspiral together with the early post-merger and highlight the differences to traditional second-order approaches at the various stages of the simulation. We find that even for finite-temperature equations of state, convergence orders higher than second order can be achieved in the inspiral and post-merger for the gravitational-wave phase. We further demonstrate that the second-order scheme overestimates the amount of proton-rich shock-heated ejecta, which can have an impact on the modelling of the dynamical part of the kilonova emission. Finally, we show that already at low resolution the growth rate of the magnetic energy is consistently resolved by using a fourth-order scheme.

Interpreting binary neutron star mergers: describing the binary neutron star dynamics, modelling gravitational waveforms, and analyzing detections

“…However, we note that even state-of-the-art simulations contain just a subset of the important physical properties influencing the postmerger and lack a clean error budget during this part of the simulation due to shocks, turbulences, or discontinuities present at or after the merger; see e.g. [388] for recent progress improving the accuracy of postmerger simulations. Independent of these caveats, the qualitative picture, as depicted in Fig.…”

“…As pointed out, there have been more than a thousand individual BNS simulations performed over the last decades, however, up to now, there is only a limited number of simulations which meet the accuracy standards necessary to develop and test existing GW models; see e.g. [63,175,182,274,294,295,388,456,531] for further discussion. The main difficulty for the production of high-quality waveforms are:…”

Spritz: a new fully general-relativistic magnetohydrodynamic code

“…To overcome such limitation, various groups implemented a local opacity calculation [62], which better accounts for non-spherical geometries. This different opacity calculation has been already employed in magnetized BNS mergers with neutrino leakage [20,21], but without accounting for neutrino heating/reabsorption. These simulations represent the current state-of-the-art in the context of magnetized BNS mergers with neutrinos.…”

“…therein). Including all of the above effects in one code is however rather challenging and only very few magnetized BNS merger simulations with neutrino treatment (via an approximate leakage scheme) have been presented so far [20,21].…”

Binary Neutron Star Merger Simulations with a Calibrated Turbulence Model

“…State-of-the-art NR simulations include a microphysical treatment of dense matter, the impact of weak reactions and neutrino radiation, and magnetic effects [63][64][65][66]. Even though modern simulations ostensibly include all of the physics believed to determine the outcome of BNS mergers, the long-term evolution of binaries after merger remains poorly known, e.g., [67].…”