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

TL;DR

This study investigates the super-orbital periodicity of M51 ULX-7, constraining the neutron star's magnetic field to approximately 2-7×10^{13} G through analysis of accretion models, light-curve periodicity, and neutron star precession.

Contribution

It provides new constraints on the neutron star's magnetic field by combining accretion theory with observed super-orbital periodicity and precession analysis.

Findings

Neutron star magnetic field estimated at 2-7×10^{13} G from spin-up and luminosity.

Super-orbital period of approximately 39 days attributed to disc precession.

Magnetic field strength estimated at 3-4×10^{13} G from neutron star precession analysis.

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.