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High luminosity accretion onto a strongly magnetized neutron star results ina radiation pressure dominated, magnetically confined accretion column. Weinvestigate the dynamics of these columns using two-dimensional radiativerelativistic magnetohydrodynamic simulations, restricting consideration tomodest accretion rates where the height of the column is low enough thatCartesian geometry can be employed. The column structure is dynamicallymaintained through high-frequency oscillations of the accretion shock at$\simeq 10-25$~kHz. These oscillations arise because it is necessary toredistribute the power released at the accretion shock through bulk verticalmotions, both to balance the cooling and to provide vertical pressure supportagainst gravity. Sideways cooling always dominates the loss of internal energy.In addition to the vertical oscillations, photon bubbles form in oursimulations and add additional spatial complexity to the column structure. Theyare not themselves responsible for the oscillations, and they do not appear toaffect the oscillation period. However, they enhance the vertical transport ofradiation and increase the oscillation amplitude in luminosity. Thetime-averaged column structure in our simulations resembles the trends instandard 1D stationary models, the main difference being that the time-averagedheight of the shock front is lower because of the higher cooling efficiency ofthe 2D column shape.

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