Jamie Bamber, Antonios Tsokaros, Milton Ruiz, and Stuart L. Shapiro
Date: October 28, 2024 arXiv:2411.00943
Abstract
The oscillation modes of neutron star (NS) merger remnants, as encoded by the kHz postmerger gravitational wave (GW) signal, hold great potential for constraining the as-yet undetermined equation of state (EOS) of dense nuclear matter. Previous works have used numerical relativity simulations to derive quasi-universal relations for the key oscillation frequencies, but most of them omit the effects of a magnetic field. We conduct full general-relativistic magnetohydrodynamics simulations of NSNS mergers with two different masses and two different EOSs (SLy and ALF2) with three different initial magnetic field topologies (poloidal and toroidal only, confined to the interior, and “pulsar-like”: dipolar poloidal extending from the interior to the exterior), with four different initial magnetic field strengths. We find that the magnetic braking and magnetic effective turbulent viscosity drives the merger remnants towards uniform rotation and increases their overall angular momentum loss. This causes the remnant to contract and the angular velocity of the quadrupole density oscillation to increase in such a way that it rotates faster than the fluid itself, in stark contrast with nonmagnetized simulations. As a result, the f2 frequency of the dominant postmerger GW mode shifts upwards over time. The overall shift is up to ∼ 200 Hz for the strongest magnetic field we consider and ∼ 50Hz for the median case and is therefore detectable in principle by future GW observatories, which should include the magnetic field in their analyses.
Nawaf Aldrees, Jamie Bamber, Jonah Doppelt, Yinuan Liang, Rohan Narasimhan, Milton Ruiz, Stuart L. Shapiro, Antonios Tsokaros, and Eric Yu