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

Abstract

We report the discovery of possible periodic X-ray dips in a pulsating ultraluminous X-ray source, M51 ULX-7, with the archival Changra observations. With ~20 days of monitoring in the superorbital descending state, we discovered three dips with separations of ~2 and ~8 days via the Bayesian block technique. A phase-dispersion minimization and a ${\chi }^2$ test suggest that the dip is likely recurrent with a period of ~2 days, consistent with the orbital period of M51 ULX-7. We interpret the dip as an obscuring of the emission from the pulsar by the vertical structure on the stream-disk interaction region or the atmosphere of the companion star. Both interpretations suggest the viewing angle to be ~60 degrees. Given that the magnetic field of M51 ULX-7 is moderately high, $B\sim 10^{13}$ G, low geometric beaming with $b\lesssim 1/2$ is sufficient to explain the observed flux and the presence of dips. Obscuration of the stellar wind remains an alternative possible origin and further monitoring of the dips will be required.

arXiv comment8 pges, 4 figures, accepted for publication in ApJ

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• The Orbit of NGC 5907 ULX-1
• Investigation of the timing and spectral properties of an Ultra-luminous X-ray pulsar NGC 7793 P13
• Spectral analysis of ultraluminous X-ray pulsars with models of X-ray pulsars
• Spectral and Temporal Properties of Ultra-luminous X-ray Source NGC 55 ULX1