Pulse Profiles of Accreting Neutron Stars from GRMHD Simulations

The pulsed X-ray emission from the neutron star surface acts as a window to study the state of matter in the neutron star interior. For accreting millisecond pulsars, the surface X-ray emission is generated from the `hotspots', which are formed as a result of magnetically channeled accretion flow hitting the stellar surface. The emission from these hotspots is modulated by stellar rotation giving rise to pulsations. Using global three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations of the star-disk system, we investigate the accretion hotspots and the corresponding X-ray pulse properties of accreting millisecond pulsars with dipolar magnetic fields. The accretion spot morphologies in our simulations are entirely determined by the accretion columns and vary as a function of the stellar magnetic inclination. For lower inclinations, the hotspots are shaped like crescents around the magnetic axis. As we increase the inclination angle, the crescents transform into elongated bars close to the magnetic pole. We model the X-ray pulses resulting from the accretion hotspots using general-relativistic ray tracing calculations and quantify the root mean square variability of the pulsed signal. The pulse amplitudes obtained from our simulations usually range between 1 - 12% rms and are consistent with the values observed in accreting millisecond pulsars. We find that the turbulent accretion flow in the GRMHD simulations introduces significant broadband variability on a timescale similar to the stellar rotational period. We also explore the impact of electron scattering absorption and show that, along with being a key factor in determining the pulse characteristics, this also introduces significant additional variability and higher harmonics in the bolometric light curve of the accreting sources.