Following a serendipitous detection with the NuSTAR observatory , we present a multi-epoch spectral and temporal analysis of an extremely bright ultraluminous X-ray source ( ULX ) located in the outskirts of the Circinus galaxy ( \sim 4 ^ { \prime } away from the nucleus ) , 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 \sim 5 in flux , and was caught in a historically bright state during our 2013 observations , with an observed 0.3–30.0 keV luminosity of 1.6 \times 10 ^ { 40 } erg s ^ { -1 } .
During this epoch , the source displayed a curved 3–10 keV spectrum , broadly similar to other bright ULXs .
Although pure thermal models result in a high energy excess in the NuSTAR data , this excess is too weak to be modelled with the disk reflection interpretation previously proposed to explain the 3–10 keV curvature in other ULXs .
In addition to flux variability , Circinus ULX5 also displays clear spectral variability .
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 relation is L \propto T ^ { 1.70 \pm 0.17 } , flatter than that expected for simple blackbody radiation .
The spectral variability displayed by Circinus ULX5 is highly reminiscent of that observed from the Galactic black hole binaries ( BHBs ) XTE J1550-564 and GRO J1655-40 at high luminosities .
This comparison would imply a black hole mass of \sim 90 \thinspace M _ { \odot } for Circinus ULX5 .
However , given the diverse behavior observed from Galactic BHB accretion disks , this mass estimate is still uncertain .
Finally , during our coordinated XMM-Newton + NuSTAR observation we find no evidence for any ionised iron absorption lines , typically associated with disk winds in Galactic BHBs .
The limits placed on any undetected features imply that we are not viewing the central regions of Circinus ULX5 through any extreme super-Eddington outflow .