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

TL;DR

This study analyzes the evolution of the spin, spectrum, and super-orbital period of the ultraluminous X-ray pulsar M51 ULX7, revealing pulsar spin-up, spectral similarities to other ULXPs, and super-orbital period variability likely due to disk precession.

Contribution

It provides the first broadband spectrum of M51 ULX7, tracks its spectral evolution with super-orbital phase, and demonstrates super-orbital period variability, supporting disk precession as the cause.

Findings

Detected pulsar spin-up at 3±0.5×10^{-10} s s^{-1} since 2018.

First high-quality broadband spectrum showing two disk-like components and a high-energy tail.

Super-orbital period increased from 38.2 to 44.2 days, indicating variability.

Abstract

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 2.8s, 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 3$\pm0.5\times10^{-10}$ 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 reexamine the super-orbital period with 3 years of Swift/XRT monitoring, finding that the period is variable, increasing from 38.2$\pm0.5$ days in 2018--2019 to 44.2$\pm0.9$ days in 2020--2021, which rules out alternative explanations for the super-orbital period.