MHD instabilities in accretion mounds - I. 2D axisymmetric simulations

We have performed stability analysis of axisymmetric accretion mounds on neutron stars in high-mass X-ray binaries by 2D magnetohydrodynamic (MHD) simulations with the PLUTO MHD code. We find that the mounds are stable with respect to interchange instabilities, but the addition of excess mass destabilizes the equilibria. Our simulations confirm that accretion mounds are unstable with respect to MHD instabilities beyond a threshold mass. We investigate both filled and hollow mounds and for the latter also compute the expected profile of cyclotron resonance scattering features (CRSF). In comparison to the CRSF from filled mounds reported in our earlier work, hollow mounds display wider and more complex line profiles. Key words: line: formation – MHD – radiation mechanisms: non-thermal – binaries: gen

Revisiting Field Burial by Accretion onto Neutron Stars

The surface magnetic field strength of millisecond pulsars (MSPs) is found to be about 4 orders of magnitude lower than that of garden variety radio pulsars (with a spin of ∼0.5–5 s and B∼1012 G). The exact mechanism of the apparent reduction of field strength in MSPs is still a subject of debate. One of the proposed mechanisms is burial of the surface magnetic field under matter accreted from a companion. In this article we review the recent work on magnetic confinement of accreted matter on neutron stars poles. We present the solutions of the magneto-static equations with a more accurate equation of state of the magnetically confined plasma and discuss its implications for the field burial mechanism

MHD instabilities in accretion mounds - II. 3D simulations

We investigate the onset of pressure-driven toroidal-mode instabilities in accretion mounds on neutron stars by 3D magnetohydrodynamic (MHD) simulations using the PLUTO MHD code. Our results confirm that for mounds beyond a threshold mass, instabilities form fingerlike channels at the periphery, resulting in mass-loss from the magnetically confined mound. Ring-like mounds with hollow interior show the instabilities at the inner edge as well. We perform the simulations for mounds of different sizes to investigate the effect of the mound mass on the growth rate of the instabilities. We also investigate the effect of such instabilities on observables such as cyclotron resonant scattering features and timing properties of such systems. Key words: line: formation – magnetic fields – MHD – methods: numerical – stars: neutro

A self-gravity module for the PLUTO code

We present a novel implementation of an iterative solver for the solution of the Poisson equation in the PLUTO code for astrophysical fluid dynamics. Our solver relies on a relaxation method in which convergence is sought as the steady-state solution of a parabolic equation, whose time-discretization is governed by the \textit{Runge-Kutta-Legendre} (RKL) method. Our findings indicate that the RKL-based Poisson solver, which is both fully parallel and rapidly convergent, has the potential to serve as a practical alternative to conventional iterative solvers such as the \textit{Gauss-Seidel} (GS) and \textit{successive over-relaxation} (SOR) methods. Additionally, it can mitigate some of the drawbacks of these traditional techniques. We incorporate our algorithm into a multigrid solver to provide a simple and efficient gravity solver that can be used to obtain the gravitational potentials in self-gravitational hydrodynamics. We test our implementation against a broad range of standard self-gravitating astrophysical problems designed to examine different aspects of the code. We demonstrate that the results match excellently with the analytical predictions (when available), and the findings of similar previous studies.Comment: Submitted to ApJS. Comments are welcom

MHD instabilities in accretion mounds on neutron star binaries

We have numerically solved the Grad-Shafranov equation for axisymmetric static MHD equilibria of matter confined at the polar cap of neutron stars. From the equilibrium solutions we explore the stability of the accretion mounds using the PLUTO MHD code. We find that pressure driven modes disrupt the equilibria beyond a threshold mound mass, forming dynamic structures, as matter spreads over the neutron star surface. Our results show that local variation of magnetic field will significantly affect the shape and nature of the cyclotron features observed in the spectra of High Mass X-ray Binaries.Comment: 4 pages, 2 figures. Contributed to proceedings of ASI conference series, 201