Zooming in and out Numerical modelling of accreting neutron stars in a general relativistic framework

Most of the stars in the universe exist in a binary system, two stars orbiting around a common center of mass. During the evolution of the binary system, if one of the stars undergoes supernova and the explosion does not disrupt the binary orbit, a two-body system consisting of a gaseous star and a compact object (black hole or neutron star) is formed. These systems consisting of a single compact object and a companion star, which experiences a mass transfer from the star to the compact object, are known as X-ray binaries. In this thesis, I focus on neutron star X-ray binaries and investigate the inflow and outflow mechanisms in these systems using different numerical methods. In chapter two, I develop a numerical setup to perform axisymmetric GRMHD simulations of accreting neutron stars with dipolar and non-dipolar magnetic fields and study different aspects such as jet formation, magnetospheric interactions, and angular momentum transport. For accreting neutron stars to be pulsating, the stellar magnetic and rotational axes need to be misaligned, and modeling inclined neutron star magnetospheres requires 3D simulations. Thus, as a next step, I perform 3D GRMHD simulations of accreting neutron stars with dipolar stellar magnetic fields in a Schwarzschild spacetime. Chapter three focuses on neutron star jet formation mechanisms and accretion dynamics in the 3D simulations. In chapter four, I study the characteristics of the accretion hotspots resulting from the 3D simulations and perform general relativistic ray-tracing calculations to investigate the X-ray pulses generated from the simulated hotspots.