M74 X-1 (CXOU J013651.1+154547): AN EXTREMELY VARIABLE ULTRALUMINOUS X-RAY SOURCE

Ultraluminous X-ray sources (ULXs) have been described variously as the most luminous normal X-ray bina-ries, hypernovae, and ‘‘intermediate-mass’ ’ black holes with masses of hundreds to thousands of solar masses. We present results onM74 X-1 (CXOU J013651.1+154547), a ULX in the nearby spiral galaxyM74 (NGC 628), from observations by Chandra and XMM-Newton. M74 X-1 is a persistent source that exhibits extreme variability and changes in spectral state on timescales of several thousand seconds. Its variability timescales and behavior resemble some Galactic microquasars. This suggests that the emission mechanism may be related to relativistically beamed jets and that M74 X-1 could be an extragalactic ‘‘microblazar’’—a microquasar whose jet axis is aligned with our line of sight. We also note that its spectrum is consistent with the presence of a low-temperature disk blackbody component, which, assuming it is due to radiation from an accretion disk, could indicate that M74 X-1 contains an intermediate-mass black hole