Observing the Extreme ULX Circinus ULX5 in an Extreme Flux State

The NuSTAR observatory recently detected a previously unidentified extreme(Lx,peak virgul 2e40 erg/s) ultraluminous Xray source (ULX) in the outskirts ofCircinus: Circinus ULX5. The current data support a simple disccorona model,reminiscent of that invoked for Galactic binaries, implying the presence of amassive (virgul90 Msun) black hole accreting around it.s Eddington limit. This is anexciting possibility, as the presence of such a black hole may pose a challengeto our current understanding of stellar evolution. We propose to test thisscenario by undertaking a short 40 ks XMM ToO observation of ULX5 in an extremelow flux state. Should this scenario be correct, the source should transition toa regular discdominated state, which can easily be identified by XMM. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]