Evolution of the spin, spectrum and super-orbital period of the ultraluminous X-ray pulsar M51 ULX7

M51 ULX7 is among a small group of known ultraluminous X-ray pulsars (ULXP). The neutron star powering the source has a spin period of 2.8 s, orbits its companion star with a period of 2 days, and a super-orbital period of 38 days is evident in its X-ray lightcurve. Here we present NuSTAR and XMM-Newton data on the source from 2019 obtained when the source was near its peak brightness. We detect the pulsations, having spun up at a rate of 4×10−10 s s−1 since they were previously detected in 2018. The data also provide the first high-quality broadband spectrum of the source. We find it to be very similar to that of other ULXPs, with two disk-like components, and a high energy tail. When combined with XMM-Newton data obtained in 2018, we explore the evolution of the spectral components with the super-orbital phase, finding that the luminosity of the hotter component drives the super-orbital flux modulation. The inclination the disk components appear to change with phase, adding support to the idea that these super-orbital periods are caused by disk precession. We also reexamine the super-orbital period with 3 years of Swift/XRT monitoring, finding that the period is variable, increasing from 38.2±0.5 days in 2018–2019 to 43.6±1.0 days in 2020–2021, which rules out alternative explanations for the super-orbital period. The change in period appears to be accompanied by an increase in the luminosity of the hot disk component with respect to the cooler one, possibly due to a reconfiguration of the accretion flow.