The X-Ray Properties of M101 ULX-1 = CXOKM101 J140332.74+542102

We report our analysis of X-ray data on M101 ULX-1, concentrating on high-state Chandra and XMM-Newton observations. We find that the high state of M101 ULX-1 may have a preferred recurrence timescale. If so, the underlying clock may have periods around 160 or 190 days, or possibly around 45 days. Its short-term variations resemble those of X-ray binaries at high accretion rates. If this analogy is correct, we infer that the accretor is a 20-40 M_solar object. This is consistent with our spectral analysis of the high-state spectra of M101 ULX-1, from which we find no evidence for an extreme (>10⁴⁰ ergs s^-1) luminosity. We present our interpretation in the framework of a high-mass X-ray binary system consisting of a B supergiant mass donor and a large stellar-mass black hole.