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

TL;DR

This paper reports the discovery of possible periodic X-ray dips in the pulsating ultraluminous X-ray source M51 ULX-7, suggesting orbital or structural causes and providing insights into the system's geometry and magnetic field.

Contribution

It presents the first evidence of recurrent X-ray dips in M51 ULX-7 and interprets their origin, offering new understanding of the system's structure and viewing angle.

Findings

Dips occur with a ~2-day period, likely orbital.

Dips are caused by obscuration from stream-disk interaction or companion star atmosphere.

The system has a magnetic field of ~10^{13} G and a viewing angle of ~60 degrees.

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, $B\sim10^{13}$ G, low geometric beaming with $b\lesssim1/2$ 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.