An Ultraluminous X-Ray Object with a 2 Hour Period in M51

Ultra-luminous X-Ray Objects are off-nucleus point sources with$L_X=10^{39}$-$10^{41}$ erg/s but the nature of such systems are largelyunidentified. Here we report a 2.1 hour period observed in a Chandra ACISobservation for ULX M51 X-7, which is located on the edge of a young starcluster in the star forming region in a spiral arm. In two ACIS observationsseparated by one year, the ULX changed from a high-hard to a low-soft spectralstate, in contrast to most Galactic low mass X-ray binaries. Based on itsperiod and spectral behaviors, we suggest that this ULX is a low mass X-raybinary system, with a dwarf companion of 0.2-0.3 $M_\odot$ and a compactaccretor, either a neutron star or a black hole, whose mass is not wellconstrained. Relativistic beaming effects are likely involved to produce theobserved high X-ray luminosities given its low accretion rate as inferred froma sustainable accretion scenario via Roche lobe overflow.Comment: accepted for ApJ Lette