A comparative analysis of standard accretion disks spectra: an application to Ultraluminous X-ray Sources

Abstract

We compare standard models of accretion disks around black holes that include the appropriate zero-torque inner boundary condition and relativistic effects on the emission and propagation of radiation. The comparison is performed adopting the multicolor disk blackbody model (MCD) as reference and looking for the parameter space in which it is in statistical agreement with "more physical" accretion disk models. We find simple 'recipes' that can be used for adjusting the estimates of the physical inner radius of the disk, the black hole mass and the accretion rate inferred using the parameters of the MCD fits. We applied these results to four ULXs for which MCD spectral fits of their X-ray soft spectral components have been published and find that, in three cases (NGC 1313 X-1, X-2 and M 81 X-9), the black hole masses inferred for a standard disk around a Schwarzschild black hole are in the interval ~100-200 solar masses. Only if the black hole is maximally rotating are the masses comparable to the much larger values previously derived in the literature.

11 pages, 8 figures. Accepted for publication in MNRAS

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• A new Ultraluminous X-ray source in the galaxy NGC 5907
• Discovery of a 6.4 h black hole binary in NGC 4490
• Spectral Analyses of the Nearest Persistent Ultraluminous X-Ray Source M33 X-8
• Swift observations of the ultraluminous X-ray source XMMU J004243.6+412519 in M31
• Searching for quantum-gravity footprint around stellar-mass black holes