GRMHD Simulations of accreting neutron stars with dipolar and complex magnetic fields

Accreting neutron stars (NS) and accreting millisecond X-ray pulsars (AMXPs) are one of the most interesting laboratories to study the physics of compact objects.  Recently, along with providing highly precise mass and radius measurements, NICER also indicted the requirement of complex magnetic field structures to explain the observed X-ray spectra in some of the isolated millisecond pulsars. Rotation-powered millisecond pulsars are assumed to have undergone significant magnetic field evolution during the accretion phase. However, the details of this process are still poorly understood. We performed a suite of GRMHD simulations of accretion onto neutron stars, contrasting dipolar with non-dipolar field structures.  Besides exploring processes in different accretion regimes, the simulations inform on the variability properties of the hotspots which can aid in future pulse-profile modeling of accreting sources.