The transient ULX in M83 that went into outburst in or shortly before 2010 is still active. Our new XMM-Newton spectra show that it has a curved spectrum typical of the upper end of the high/soft state or slim-disk state. It appears to be spanning the gap between Galactic stellar-mass black holes and the ultraluminous state, at X-ray luminosities ≈(1-3)×1039 erg s−1 (a factor of two lower than in the 2010-2011 Chandra observations). From its broadened disk-like spectral shape at that luminosity, and from the fitted inner-disk radius and temperature, we argue that the accreting object is an ordinary stellar-mass black hole with M∼10-20M⊙. We suggest that in the 2010-2011 Chandra observations, the source was seen at a higher accretion rate, resulting in a power-law-dominated spectrum with a soft excess at large radii.
@article{arxiv.1411.7212,
title = {The slim-disk state of the ultraluminous X-ray source in M83},
author = {Roberto Soria and K. D. Kuntz and Knox S. Long and William P. Blair and Paul P. Plucinsky and P. Frank Winkler},
journal= {arXiv preprint arXiv:1411.7212},
year = {2015}
}