Radiative Relativistic MHD Simulations of Neutron Star Column Accretion in Cartesian Geometry

Strongly magnetized neutron stars in X-ray binaries accreting at supercritical luminosities form radiation pressure dominated accretion columns near their magnetic poles. Such columns are known to be intrinsically unstable and therefore cannot form stationary structures. We perform radiative relativistic MHD simulations to investigate the dynamics of these columns. We find that the overall vertical structure is maintained through nonlinear oscillations, regardless of the existence of the photon bubble instability. The oscillations are mainly driven by an imbalance between gravity and radiation support in the sinking region, where the accretion power would otherwise fail to replenish the radiative losses. The existence of this oscillatory behavior therefore facilitates the redistribution of the accretion power to maintain the column structure. When the photon bubble instability is resolved in the simulation, an incoherent spatial structure forms below the shock front. Such a structure creates cavities at the shock front and allows the radiation to penetrate the shock front, which leads to the formation of a secondary shock structure above. However, this complicated dynamical effect seems not to alter the oscillation period. In the future, we will post-process these simulations to predict spectral, polarization, and timing observables. We are also pursuing more global simulations in order to examine if the above dynamics still holds.