Photon Bubble Oscillations in Accretion-powered Pulsars

We describe time-dependent, two-dimensional axisymmetric radiation hydrodynamic calculations of locally super-Eddington accretion onto highly magnetized neutron stars appropriate to the flow onto the polar caps of high-luminosity X-ray pulsars. Our calculations show the development and nonlinear evolution of photon bubble instabilities in the settling plasma below the radiation-dominated shock that terminates free fall along the magnetic field. The photon bubbles develop as elongated, very low density, optically thin regions of local outflow forming typically a few kilometers above the neutron star's surface with outflow velocities of order 0.1 c . These "holes" in the plasma reduce the total opacity of the accretion mound and through photon advection significantly enhance the efficiency of radiation transport. The number of bubbles varies from a few to 20 and results in significant fluctuations (4%-10%) in the emitted luminosity. These fluctuations appear as reasonably high quality ( Q ) "photon bubble oscillations" (PBOs) in the power spectrum of the emergent luminosity time series on timescales between 0.1 and 1 ms, a phenomenon not previously observed. The discovery of PBOs would provide a powerful probe into the physical characteristics of super-Eddington flows. We use our results to exhibit power spectra of the predicted luminosity oscillations and discuss the feasibility of their detection by the X - Ray Timing Explorer ( XTE ). We also use our results to calculate the emergent spectrum and show that it is a highly depleted modified blackbody distribution roughly consistent with known rotation phase-averaged observations in the 10-100 keV range. Thus we have shown that the spectra of X-ray pulsars are a natural consequence of the dynamics of polar cap accretion.