The Slim-disk State of the Ultraluminous X-Ray Source in M83

Abstract

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 (BHs) and the ultraluminous state, at X-ray luminosities of ≈1-3 × 10³⁹ erg s^-1 (a factor of two lower than in the 2010 and 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 BH with M ~ 10-20 M _⊙. We suggest that in the 2010 and 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.