The Birth of an Ultra-Luminous X-ray Source in M83

TL;DR

This paper reports the discovery and analysis of a new ultraluminous X-ray source in galaxy M83, showing evidence of a black hole accreting from a low-mass star, with implications for understanding ULX populations.

Contribution

It presents the first detection of a ULX with a low-mass donor star, challenging the assumption that ULX optical counterparts are always high-mass companions.

Findings

The ULX has remained ultraluminous over a year with flux variation of a factor of two.

Spectral analysis suggests accretion onto a black hole of 40-100 solar masses.

Optical observations indicate a low-mass donor star with an irradiated accretion disk.

Abstract

A previously undetected X-ray source (L_X<10**36 erg/s) in the strongly star-forming galaxy M83 entered an ultraluminous state between August 2009 and December 2010. It was first seen with Chandra on 23 December 2010 at L_X ~ 4 10**39 ergs/s, and has remained ultraluminous through our most recent observations in December 2011, with typical flux variation of a factor of two. The spectrum is well fitted by a combination of absorbed power-law and disk black-body models. While the relative contributions of the models varies with time, we have seen no evidence for a canonical state transition. The luminosity and spectral properties are consistent with accretion powered by a black hole with M_BH ~ 40-100 solar masses. In July 2011 we found a luminous, blue optical counterpart which had not been seen in deep HST observations obtained in August 2009. These optical observations suggest that the donor star is a low-mass star undergoing Roche-lobe overflow, and that the blue optical emission seen during the outburst is coming from an irradiated accretion disk. This source shows that ultraluminous X-ray sources (ULXs) with low-mass companions are an important component of the ULX population in star-forming galaxies, and provides further evidence that the blue optical counterparts of some ULXs need not indicate a young, high-mass companion, but rather that they may indicate X-ray reprocessing.