2007ApJ...667..395F

The nature of ultraluminous X-ray sources (ULXs) in nearby galaxies is one of the major open questions in modern X-ray astrophysics. One possible explanation for these objects is an inhomogeneous, radiation-dominated accretion disk around a ∼10 M_☉ black hole, the so-called photon bubble model. While previous studies of this model have focused primarily on its radiation hydrodynamics aspects, in this paper, we provide an analysis of its X-ray spectral (continuum and possible edge and line) characteristics. Compton reflection between high- and low-density regions in the disk may provide the key to distinguishing this model from others, such as accretion onto an intermediate-mass black hole. We couple a Monte Carlo-Fokker-Planck radiation transport code with the XSTAR code for reflection to simulate the photon spectra produced in a photon bubble model for ULXs. We find that reflection components tend to be very weak and in most cases not observable, and we make predictions for the shape of the high-energy Comptonizing spectra. In many cases, the Comptonization dominates the spectra even down to approximately a few keV. In one simulation, a ∼9 keV feature was found, which may be considered a signature of photon bubbles in ULXs; furthermore, we make predictions of high-energy power laws which may be observed by future instruments.