GRMHD simulations of accreting neutron stars I: Non-rotating dipoles

Dr. Sercan Çıkıntoğlu, Istanbul Technical University, Faculty of Science and Letters, Physics Engineering Department, and Institut für Theoretische Physik, Goethe-Universität, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, Germany

Prof. Dr. K. Yavuz Ekşi, Istanbul Technical University, Faculty of Science and Letters, Physics Engineering Department

Prof. Dr. Luciano Rezzolla, Institut für Theoretische Physik, Goethe-Universität, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, Germany, Frankfurt Institute for Advanced Studies, Ruth-Moufang-Str. 1, 60438 Frankfurt am Main, Germany, School of Mathematics, Trinity College, Dublin 2, Ireland

Some neutron stars live in low-mass X-ray binaries which consist of a neutron star and a companion object (mass donor). In these systems, the companion star lost mass via Roche-lobe overflow and the overflowing matter forms an accretion disc if its angular momentum is high enough. The matter may accrete onto the neutron star from the disc and the source may appear as an X-ray pulsar powered by the gravitational potential energy of the accreting matter. The details of this phenomenon depend on the disc-magnetosphere interactions.

The dynamics of the accreting flow is governed by the magnetohydrodynamics equations which are eight non-linear partial differential equations. Because of the complex nature of these equations, numerical simulations are required to understand these sources. Also, general relativistic effects have to be considered since neutron stars are compact objects. With this motivation, we performed ten general relativistic magnetohydrodynamics simulations (GRMHD) for a non-rotating magnetised neutron star in our paper entitled “GRMHD simulations of accreting neutron stars I: Non-rotating dipoles” (https://arxiv.org/abs/2204.12275 for the preprint version and 10.1093/mnras/stac2510 for the published version in Monthly Notices of the Royal Astronomical Society).

We employ BHAC (Black Hole Accretion Code) to perform simulations. We follow the evolution of a geometrically thick matter torus driven into accretion by the development of a magnetorotational instability.  By analyzing the results of simulations, we determine the location of the magnetospheric radius and its dependency on the magnetic moment and mass-accretion rate. As a result, we find that the magnetospheric radius scales with 4/7th power of the magnetic moment as in the conventional models, however, its dependence on the accretion rate is very weak.  Furthermore, we find that the material torque correlates linearly with the mass-accretion rate and both of them exhibit fluctuations. Moreover, the total torque shows large fluctuations in strong magnetic field simulations which can be associated with the spin fluctuations observed in X-ray pulsars.

This work is done by Dr. Sercan Çıkıntoğlu (İTÜ),  Prof. Dr. K. Yavuz Ekşi (İTÜ), and Prof. Dr. Luciano Rezzolla (Goethe University).