  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.
