Photon Bubble Oscillations in Accretion-powered Pulsars

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Abstract
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 0.1c. 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.