The ultraluminous X-ray source NGC 5643 ULX1: a large stellar mass black
  hole accreting at super-Eddington rates?
  A sub-set of the brightest ultraluminous X-ray sources (ULXs), with X-ray
luminosities well above $10^{40}$ erg s$^{-1}$, typically have energy spectra
which can be well described as hard power-laws, and short-term variability in
excess of $\sim10\%$. This combination of properties suggests that these ULXs
may be some of the best candidates to host intermediate mass black holes
(IMBHs), which would be accreting at sub-Eddington rates in the hard state seen
in Galactic X-ray binaries. In this work, we present a temporal and spectral
analysis of all of the available XMM-Newton data from one such ULX, the
previously poorly studied 2XMM J143242.1$-$440939, located in NGC 5643. We
report that its high quality EPIC spectra can be better described by a broad,
thermal component, such as an advection dominated disc or an optically thick
Comptonising corona. In addition, we find a hint of a marginal change in the
short-term variability which does not appear to be clearly related to the
source unabsorbed luminosity. We discuss the implications of these results,
excluding the possibility that the source may be host an IMBH in a low state,
and favouring an interpretation in terms of super-Eddington accretion on to a
black hole of stellar origin. The properties of NGC 5643 ULX1 allow us to
associate this source to the population of the hard/ultraluminous ULX class.
