UC Santa Barbara

Accretion onto a strongly magnetized neutron star at a sufficiently high mass accretion rate results in the formation of a radiation pressure-supported columnar structure near the polar regions. In this region, the accretion inflow is magnetically constrained and shocked above the stellar surface. Below the shock, the accretion column liberates most of the accretion power through the sideways radiation emission in a so-called `fan-beam' pattern, in contrast to the `pencil-beam' emission where radiation leaves directly from the top of the column. The physics of the accretion column plays a defining role in understanding the observations of accretion-powered X-ray pulsars, including pulsating ultraluminous X-ray sources (ULXs). The observed pulsations arise from the misalignment between the anisotropic radiation emission and the spin axis of the rotating neutron star. We perform radiative relativistic MHD simulations to study the nonlinear dynamics of the accretion column. The column structure is extremely dynamical and exhibits kHz quasi-periodic oscillations. The existence of the photon bubble instability is identified in simulated accretion columns but proved to be not responsible for triggering the oscillatory behaviors. Instead, the oscillations originate from the inability of the system to resupply heat and locally balance the sideways cooling. When the oscillation amplitude is sufficiently large, the emergent radiation can exhibit hybrid fan- and pencil-beam patterns. The column structure is very sensitive to the shock geometry, which directly determines the cooling efficiency. A more diverging geometry of the accretion column can provide more heat support through PdV work. The time-averaged column structures from the simulations can be approximately reproduced by a 1D stationary model, given the correction for the actual 2D mound shape of the time-averaged column. The increase in magnetic opacity with temperature below the radiative shock may introduce an additional unstable mechanism in the dynamics of the accretion column. Pair production can boost the opacity above ~4e8 K near the base of the column, which is likely to introduce further dynamical effects. To further investigate these problems, we propose an extension of the current numerical framework by incorporating magnetic polarization into the radiation module.