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Following a serendipitous detection with the NuSTAR observatory, we present amulti-epoch spectral and temporal analysis of an extreme ultraluminous X-raysource (ULX) located in the outskirts of the Circinus galaxy, hereafterCircinus ULX5, including coordinated follow-up observations with XMM-Newton andNuSTAR. The NuSTAR data presented here represent one of the first instances ofa ULX reliably detected at hard (E>10 keV) X-rays. Circinus ULX5 is variable onlong timescales by at least a factor of ~5 in flux, and was caught in ahistorically bright state during our 2013 observations (0.3-30.0 keV luminosityof 1.6e40 erg/s). During this epoch, the source displayed a curved 3-10 keVspectrum, broadly similar to other bright ULXs. While in many cases theinterpretation of spectral components in ULXs is uncertain, the spectral andtemporal properties of the all the high quality datasets currently availablestrongly support a simple disk-corona model reminiscent of that invoked forGalactic binaries, with the accretion disk becoming more prominent as theluminosity increases. However, although the disk temperature and luminosity areremarkably well correlated across all timescales currently probed, the observedluminosity follows T^(1.70+/-0.17), flatter than that expected for simpleblackbody radiation. The spectral variability displayed by Circinus ULX5 ishighly reminiscent of that observed from some known Galactic black holebinaries (BHBs) at high luminosities. This comparison would imply a black holemass of ~90 Msun for Circinus ULX5. However, given the diverse behaviorobserved from Galactic BHB accretion disks, this mass estimate is stilluncertain. Finally, the limits placed on any undetected iron absorptionfeatures with the 2013 dataset imply that we are not viewing the centralregions of Circinus ULX5 through any extreme super-Eddington outflow.[abridged]

An Extremely Luminous and Variable Ultraluminous X-ray Source in the Outskirts of Circinus Observed with NuSTAR