Dynamics of Neutron Star Accretion Columns

Accretion powered X-ray pulsars are neutron stars that accrete matter from a companion star in a binary system.  The strong magnetic field of the neutron star guides the material onto the magnetic poles of the star, which are generally misaligned from the rotation axis of the star, thereby resulting in pulsating X-rays as the star spins relative to an observer. At sufficiently high accretion rates, the magnetically confined accretion flow can form a column structure near the magnetic poles in which radiation pressure supports the column against gravity.  Such structures are known to suffer from the photon bubble instability and therefore cannot be stationary.  We perform radiative relativistic MHD simulations to investigate the nonlinear dynamics of the accretion column.  The column structure is highly dynamical and displays quasi-periodic oscillations at frequencies ranging from 5 to 25 kHz. The oscillation is mainly driven by the global imbalance between gravity and radiation support due to inefficient heat transport. The instability does not destroy the column structure, but instead drives the gas to oscillate along the magnetic field, which increases the efficiency of global heat transport and facilitates the maintenance of the column structure.