Title: An Ultra-luminous X-Ray Object with a 2 hour period in M51

Abstract: 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 largely unidentified. Here we report a 2.1 hour period observed in a Chandra ACIS observation for ULX M51 X-7, which is located on the edge of a young star cluster in the star forming region in a spiral arm. In two ACIS observations separated by one year, the ULX changed from a high-hard to a low-soft spectral state, in contrast to most Galactic low mass X-ray binaries. Based on its period and spectral behaviors, we suggest that this ULX is a low mass X-ray binary system, with a dwarf companion of 0.2-0.3 $M_\odot$ and a compact accretor, either a neutron star or a black hole, whose mass is not well constrained. Relativistic beaming effects are likely involved to produce the observed high X-ray luminosities given its low accretion rate as inferred from a sustainable accretion scenario via Roche lobe overflow.