Revisiting Putative Cool Accretion Disks in Ultraluminous X-ray Sources
Abstract
Soft, potentially thermal spectral components observed in some ULXs can be fit with models for emission from cool, optically-thick accretion disks. If that description is correct, the low temperatures that are observed imply accretion onto "intermediate-mass" black holes. Subsequent work has found that these components may follow an inverse relationship between luminosity and temperature, implying a non-blackbody origin for this emission. We have re-analyzed numerous XMM-Newton spectra of extreme ULXs. Crucially, observations wherein the source fell on a chip gap were excluded owing to their uncertain flux calibration, and the neutral column density along the line of sight to a given source was jointly determined by multiple spectra. The luminosity of the soft component is found to be positively correlated with temperature, and to be broadly consistent with L ~ T^4 in the measured band pass, as per blackbody emission from a standard thin disk. These results are nominally consistent with accretion onto black holes with masses above the range currently known in Galactic X-ray binaries, though there are important caveats. Emission from inhomogeneous or super-Eddington disks may also be consistent with the data.
Keywords
Cite
@article{arxiv.1309.4350,
title = {Revisiting Putative Cool Accretion Disks in Ultraluminous X-ray Sources},
author = {J. M. Miller and D. J. Walton and A. L. King and M. T. Reynolds and A. C. Fabian and M. C. Miller and R. C. Reis},
journal= {arXiv preprint arXiv:1309.4350},
year = {2015}
}
Comments
Accepted for publication in ApJ Letters