Chandra probes the X-ray variability of M51 ULX-7: evidence of propeller transition and X-ray dips on orbital periods

TL;DR

This study analyzes the X-ray variability of M51 ULX-7, revealing evidence of a propeller transition, X-ray dips related to orbital periods, and insights into the system's geometry and accretion states.

Contribution

It provides new observational evidence of propeller transition and orbital dips in ULX pulsar M51 ULX-7, enhancing understanding of its accretion and geometric configurations.

Findings

Extended low flux state suggests propeller transition.

Discovery of periodic X-ray dips linked to orbital period.

Implication of an edge-on binary orientation.

Abstract

We report on the temporal properties of the ULX pulsar M51 ULX-7 inferred from the analysis of the 2018-2020 Swift/XRT monitoring data and archival Chandra data obtained over a period of 33 days in 2012. We find an extended low flux state, which might be indicative of propeller transition, lending further support to the interpretation that the NS is rotating near equilibrium. Alternatively, this off state could be related to a variable super-orbital period. Moreover, we report the discovery of periodic dips in the X-ray light curve that are associated with the binary orbital period. The presence of the dips implies a configuration where the orbital plane of the binary is closer to an edge on orientation, and thus demonstrates that favorable geometries are not necessary in order to observe ULX pulsars.These characteristics are similar to those seen in prototypical X-ray pulsars like Her X-1 and SMC X-1 or other ULX pulsars like NGC 5907 ULX1.