Institute for Advanced Study Astrophysics Seminar

Accretion-powered X-ray pulsars are neutron stars that accrete matter from a companion star in a binary system, which exhibit fascinating dynamics. The strong magnetic field of the neutron star guides material onto the magnetic poles, which are generally misaligned from the spin axis, resulting in pulsating X-rays as the star rotates. At high accretion rates, a magnetically confined accretion flow forms a column structure near the magnetic poles, supported against gravity by radiation pressure. This column structure is highly dynamical, displaying kHz quasi-periodic oscillations. Simulated accretion columns reveal the presence of the photon bubble instability, although it is not the trigger for the oscillatory behaviors. Instead, the oscillations originate from the inability of the system to resupply heat and locally balance sideways cooling. The column structure is very sensitive to the shock geometry because it directly determines the cooling efficiency. The simulated time-averaged column structures can be approximately reproduced by a corrected 1D stationary model, accounting for the actual 2D/3D shape of the time-averaged column. The scattering opacity can be significantly reduced in a strong magnetic field, potentially altering the column structure and variability. However, pair production appears to boost the opacity above ~4e8 K and would likely introduce additional effects.