MHD instabilities in accretion mounds on neutron star binaries

Dipanjan Mukherjee1*, Dipankar Bhattacharya1 and Andrea Mignone2
1Inter University Centre for Astronomy & Astrophysics, Pune 411 007, India
2Dipartimento di Fisica Generale, Universita di Torino, Torino, Italy

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We have numerically solved the Grad-Shafranov equation for axisymmetric static MHD equilibria of matter confined to 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.

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Keywords : accretion – instabilities – (magnetohydrodynamics) MHD – stars: neutron – (stars:) binaries: general