Extreme ULXs: super-Eddington accretors, or intermediate-mass black holes?

Ultraluminous X-ray sources (ULXs) with luminosities up to 2 imes 10(40) erg s(-1) are now regarded as likely super-Eddington accretors powered by stellar mass black holes, and those above 10(41) erg s(-1) may be good intermediate-mass black hole (IMBH) candidates. However, the few objects lying between these ranges have an uncertain nature, and remain poorly studied. Here, we propose a new deep observation of one such object located within 20 Mpc. We will use the high quality EPIC data to determine whether its spectrum shows the characteristic signatures of the ultraluminous state, indicative of super-Eddington accretion onto large stellar black holes, or whether it appears consistent with the hard power-law spectra and > 10% fractional variability of an IMBH in the hard state.