Speaker
Description
Neutron stars are known to have two distinct populations in terms of their surface magnetic fields, viz. low magnetic fields of $10^8–10^{10}$ G such as in millisecond pulsars, and those with higher magnetic fields ($10^{12}$ G), comprising of the bulk of known neutron star population. An accretion induced reprocessing scenario is often invoked to explain the higher spin frequencies of millisecond pulsars. However, the reason for the low magnetic field of millisecond pulsars is still an open problem. In a new work Yeole et al. (2025), we explore the mechanism of surface magnetic field burial inside the neutron star due to accreted matter. The accreted matter accumulated near the magnetic poles forms an accretion mountain in which the screening currents are responsible for magnetic field burial. We calculate the magnetic field geometry of the neutron star above the surface or ocean using Grad Shafranov equation. We have developed a self consistent way of accounting for the reduction in the surface magnetic field using a multipolar current free boundary condition. We have explored several types of accretion mountain configurations in this work, including accretion on pre-existing ocean and also multi-polar surface magnetic fields. Furthermore, I will present new results of 2D and 3D MHD simulations of these accretion mountains performed using a newly implemented SemiRMHD solver in PLUTO, along with a force-free magnetosphere. The simulations allow us to explore the onset of possible MHD instabilities in such mountains, their non-linear growth and cross-field spreading of matter, beyond their initial confinement.