A 106 Day Period in the Nuclear Source X-8 in M33

With an X-ray luminosity of about 1039 ergs s−1, the source X-8, coincident with the optical center of M33, is the most luminous X-ray source in the Local Group. However, its nature remains a mystery. We present here new and archival ROSAT observations of X-8 spread over 6 yr that show variability and a periodicity of ~106 days. This implies that (most of) the emission from M33 X-8 arises from a single object, perhaps a binary system with a black hole primary of ~10 M☉. We suggest that the companion is a giant orbiting with a period of ~10 days and that the observed modulation is "superorbital," analogous to that seen in Cyg X-2 and X1820-30.