Structure and stability of magnetically confined mounds in a Neutron star

Abstract

A high-mass X-ray binary (HMXB) is a binary system of a compact object, neutron star or black hole and a companion high mass star. The compact object accretes from its companion and the system emits strongly in X-ray. The proposed thesis project deals with an HMXB with neutron star as the compact object. The infalling matter from the companion star is obstructed by the magnetosphere of the neutron star and is guided by the magnetic field lines leading up to the poles, forming an accretion column along the magnetic field. This accretion column deposits the matter on the neutron star surface as a result of which an accretion mound is formed at each magnetic pole of the strongly magnetized neutron star. The mound distorts the local magnetic field, the imprint of which can be observed via Cyclotron Resonance Scattering Features (CRSF) in the X-ray spectrum. The spreading of matter onto the neutron star surface from the magnetically confined accretion mound has so far been poorly understood. How much matter can be retained in the mound and column is decided by the interchange instabilities - such as ballooning instability, fluting instability and Parker instability - as well as dissipative processes such as ohmic diffusion and magnetic reconnection. Numerical investigation of some of these processes is attempted in this thesis. The matter accumulated in the mound adds a quadrupole moment to the mass distribution of the star, which then leads to the generation of gravitational waves as the star spins. The amplitude of these waves will be determined by the amount of mass in the mound and its density distribution, both critically dependent on the instabilities and dissipative processes. So, we try to understand the structure and stability of the magnetically confined accretion mounds formed at the poles of the neutron star.