Energy flow and radiation efficiency in radiative GRMHD simulations of neutron star ultraluminous X-ray sources

TL;DR

This study uses radiative GRMHD simulations to analyze how magnetic field strength and accretion rate affect energy flow and radiation efficiency in neutron star ULXs, revealing key dependencies and beaming effects.

Contribution

It provides new insights into the impact of magnetic dipole strength and accretion rate on energy efficiency and outflow power in neutron star ULXs through detailed simulations.

Findings

Stronger magnetic dipoles reduce radiation efficiency.

Weaker magnetic dipoles produce more beaming and higher apparent luminosity.

Higher accretion rates increase outflow power and kinetic efficiency.

Abstract

We investigate numerically the energy flow and radiation efficiency of accreting neutron stars as potential ultraluminous X-ray sources (ULXs). We perform ten simulations {in radiative general relativistic magnetohydrodynamics (GRRMHD)}, exploring six different magnetic dipole strengths ranging from 10 to 100 GigaGauss, along with three accretion rates, 100, 300, and 1000 Eddington luminosity units. Our results show that the energy efficiency in simulations with a strong magnetic dipole of 100 GigaGauss is approximately half that of simulations with a magnetic dipole an order of magnitude weaker. Consequently, radiation efficiency is lower in simulations with stronger magnetic dipoles. We also demonstrate that outflow power increases as the magnetic dipole weakens, resulting in stronger beaming in simulations with weaker magnetic dipoles. As a result of beaming, simulations with magnetic dipole strengths below 30 GigaGauss exhibit apparent luminosities consistent with those observed in ULXs. As for the accretion rates, we find that higher accretion rates lead to more powerful outflows, higher kinetic efficiency, and lower radiation efficiency compared to those of lower accretion rate simulations.