An Extremely Luminous and Variable Ultraluminous X-ray Source in the
  Outskirts of Circinus Observed with NuSTAR
  Following a serendipitous detection with the NuSTAR observatory, we present a
multi-epoch spectral and temporal analysis of an extreme ultraluminous X-ray
source (ULX) located in the outskirts of the Circinus galaxy, hereafter
Circinus ULX5, including coordinated follow-up observations with XMM-Newton and
NuSTAR. The NuSTAR data presented here represent one of the first instances of
a ULX reliably detected at hard (E>10 keV) X-rays. Circinus ULX5 is variable on
long timescales by at least a factor of ~5 in flux, and was caught in a
historically bright state during our 2013 observations (0.3-30.0 keV luminosity
of 1.6e40 erg/s). During this epoch, the source displayed a curved 3-10 keV
spectrum, broadly similar to other bright ULXs. While in many cases the
interpretation of spectral components in ULXs is uncertain, the spectral and
temporal properties of the all the high quality datasets currently available
strongly support a simple disk-corona model reminiscent of that invoked for
Galactic binaries, with the accretion disk becoming more prominent as the
luminosity increases. However, although the disk temperature and luminosity are
remarkably well correlated across all timescales currently probed, the observed
luminosity follows T^(1.70+/-0.17), flatter than that expected for simple
blackbody radiation. The spectral variability displayed by Circinus ULX5 is
highly reminiscent of that observed from some known Galactic black hole
binaries (BHBs) at high luminosities. This comparison would imply a black hole
mass of ~90 Msun for Circinus ULX5. However, given the diverse behavior
observed from Galactic BHB accretion disks, this mass estimate is still
uncertain. Finally, the limits placed on any undetected iron absorption
features with the 2013 dataset imply that we are not viewing the central
regions of Circinus ULX5 through any extreme super-Eddington outflow.
[abridged]
