  We perform 2D axisymmetric radiative relativistic MHD simulations of
radiation pressure supported neutron star accretion columns in split-monopole
magnetic fields. The accretion columns exhibit quasi-periodic oscillations,
which manifest in the luminosity power spectrum as 2-10 kHz peaks, together
with broader extensions to somewhat higher frequencies. The peak frequency
decreases for wider columns or higher mass accretion rates. In contrast to the
case of shorter columns in uniform magnetic fields, pdV work contributes
substantially to maintaining the radiation pressure inside the column against
sideways radiative cooling. This is in part due to the compression associated
with accretion along the converging magnetic field lines towards the stellar
surface. Propagating entropy waves which are associated with the slow-diffusion
photon bubble instability form in all our simulations. Radial advection of
radiation from the oscillation itself as well as the entropy waves is also
important in maintaining radiation pressure inside the column. The
time-averaged profile of our fiducial simulation accretion is approximately
consistent with the classical 1D stationary model provided one incorporates the
correct column shape. We also quantify the porosity in all our accretion column
simulations so that this may also in principle be used to improve 1D models.
