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


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Çıkıntoğlu S., Ekşi K. Y. , Rezzolla L.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, vol.517, no.3, pp.3212-3226, 2022 (SCI-Expanded) identifier identifier

  • Publication Type: Article / Article
  • Volume: 517 Issue: 3
  • Publication Date: 2022
  • Doi Number: 10.1093/mnras/stac2510
  • Journal Name: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Academic Search Premier, Aerospace Database, Applied Science & Technology Source, Communication Abstracts, Compendex, INSPEC, Metadex, zbMATH, DIALNET, Civil Engineering Abstracts
  • Page Numbers: pp.3212-3226
  • Keywords: accretion, accretion discs, MHD, methods: numerical, stars: neutron, X-RAY SOURCES, DISK ACCRETION, ANGULAR-MOMENTUM, SPIN-DOWN, MAGNETOHYDRODYNAMIC SIMULATIONS, MAGNETIZED STARS, TORQUE REVERSAL, MILLISECOND PULSAR, CENTAURUS X-3, BLACK-HOLES
  • Istanbul Technical University Affiliated: Yes

Abstract

We study the general-relativistic dynamics of matter being accreted on to and ejected by a magnetized and non-rotating neutron star. The dynamics is followed in the framework of fully general relativistic magnetohydrodynamics (GRMHD) within the ideal-MHD limit and in two spatial dimensions. More specifically, making use of the numerical code BHAC, we follow the evolution of a geometrically thick matter torus driven into accretion by the development of a magnetorotational instability. By making use of a number of simulations in which we vary the strength of the stellar dipolar magnetic field, we can determine self-consistently the location of the magnetospheric (or Alfven) radius r(msph) and study how it depends on the magnetic moment mu and on the accretion rate. Overall, we recover the analytic Newtonian scaling relation, i.e. r(msph) proportional to B-4/7, but also find that the dependence on the accretion rate is very weak. Furthermore, we find that the material torque correlates linearly with the mass-accretion rate, although both of them exhibit rapid fluctuations. Interestingly, the total torque fluctuates drastically in strong magnetic field simulations and these unsteady torques observed in the simulations could be associated with the spin fluctuations observed in X-ray pulsars.