The slim-disk state of the ultraluminous X-ray source in M83

TL;DR

This paper analyzes the spectral state of the ultraluminous X-ray source in M83, revealing it to be in a slim-disk state consistent with a stellar-mass black hole at high accretion rates.

Contribution

It provides new XMM-Newton spectral data showing the source's slim-disk state and estimates the black hole mass, bridging the gap between stellar-mass black holes and ultraluminous states.

Findings

The source exhibits a curved spectrum typical of the slim-disk state.

The black hole mass is estimated to be 10-20 solar masses.

The source's luminosity is lower than during previous observations, indicating a different accretion state.

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 and the ultraluminous state, at X-ray luminosities $\approx (1$-$3) \times 10^{39}$ 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 \sim$$10$-$20 M_{\odot}$. 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.