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

Abstract

M51 ULX7 is among a small group of known ultra luminous X-ray pulsars (ULXPs). 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 light curve. 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 3 ? 0.5 ? 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 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, which may support the idea that these super orbital periods are caused by disk precession. We also re examine the super orbital period with 3 yr of Swift/XRT monitoring, finding that the period is variable, increasing from 38.2 ? 0.5 days in 2018-2019 to 44.2 ? 0.9 days in 2020-2021, which rules out alternative explanations for the super orbital period.