Possible Periodic Dips in the Pulsating Ultraluminous X-Ray Source M51 ULX-7

We report the discovery of possible periodic X-ray dips in a pulsating ultraluminous source, M51 ULX-7, with archival Changra observations. With ~20 days monitoring superorbital descending state, we discovered three separations ~2 and ~8 via Bayesian block technique. A phase-dispersion minimization $\chi^2$ test suggest that dip is likely recurrent period days, consistent orbital ULX-7. interpret as an obscuring emission from pulsar by vertical structure on stream-disk interaction region or atmosphere companion star. Both interpretations viewing angle to be ~60 degrees. Given magnetic field ULX-7 moderately high, $B\sim10^{13}$ G, low geometric beaming $b\lesssim1/2$ sufficient explain observed flux presence dips. Obscuration stellar wind remains alternative origin further will required.