Detailed atmosphere model fits to disk-dominated ULX spectra

We have chosen six ultraluminous X-ray sources from the XMM-Newton archive whose spectra have high signal-to-noise ratio (S/N) and can be fitted solely with a disk model without requiring any power-law component. To estimate systematic errors in the inferred parameters, we fit every spectrum to two different disk models, one based on local blackbody emission (KERRBB) and one based on detailed atmosphere modeling (BHSPEC). Both incorporate full general relativistic treatment of the disk surface brightness profile, photon Doppler shifts, and photon trajectories. We found in every case that they give almost identical fits and similar acceptable parameters. The best-fit value of the most interesting parameter, the mass of the central object, is between 23 and 73 M-circle dot in five of the six examples. In every case, the best-fit inclination angle and mass are correlated, in the sense that large mass corresponds to high inclination. Even after allowing for this degeneracy, we find that, with greater than or similar to 99.9% formal statistical confidence, three of the six objects have mass greater than or similar to 25 M-circle dot; for the other three, these data are consistent with a wide range of masses. A mass greater than several hundred M-circle dot is unlikely for the three best-constrained objects. These fits also suggest comparatively rapid black hole spin in the three objects whose masses are relatively well determined, but our estimate of the spin is subject to significant systematic error having to do with uncertainty in the underlying surface brightness profile.