M51 ULX-7: super-orbital periodicity and constraints on the neutron star magnetic field

Abstract

In the current work we explore the applicability of standard theoretical models of accretion to the observed properties of M51 ULX-7. The spin-up rate and observed X-ray luminosity are evidence of a neutron star with a surface magnetic field of $2-7\times10^{13}$ G, rotating near equilibrium. Analysis of the X-ray light-curve of the system (Swift/XRT data) reveals the presence of a $\sim$39 d super-orbital period. We argue that the super-orbital periodicity is due to disc precession, and that material is accreted onto the neutron star at a constant rate throughout it. Moreover, by attributing this modulation to the free precession of the neutron star we estimate a surface magnetic field strength of $3-4\times10^{13}$ G. The agreement of these two independent estimates provide strong constraints on the surface polar magnetic field strength of the neutron star.