NuSTAR J095551+6940.8: a highly magnetised neutron star with super-Eddington mass accretion

TL;DR

This paper models the super-Eddington accretion in the ultraluminous X-ray source M82 X-2, proposing it as a highly magnetized neutron star with a magnetic field around 10^{13} G, explaining observed spin variations and luminosity.

Contribution

It introduces a numerical model of accreting pulsars within the magnetically threaded-disk framework, identifying the magnetic field strength that best explains observations of M82 X-2.

Findings

The source likely has a magnetic field of ~10^{13} G.

The neutron star is probably in the propeller phase during archival observations.

Moderate beaming (0.5 to 1) is consistent with the data.

Abstract

The identification of the Ultraluminous X-ray source (ULX) X-2 in M82 as an accreting pulsar has shed new light on the nature of a subset of ULXs, while rising new questions on the nature of the super-Eddington accretion. Here, by numerically solving the torque equation of the accreting pulsar within the framework of the magnetically threaded-disk scenario, we show that three classes of solutions, corresponding to different values of the magnetic field, are mathematically allowed. We argue that the highest magnetic field one, corresponding to B $\sim 10^{13}$ G, is favoured based on physical considerations and the observed properties of the source. In particular, that is the only solution which can account for the observed variations in $\dot{P}$ (over four time intervals) without requiring major changes in $\dot{M}$, which would be at odds with the approximately constant X-ray emission of the source during the same time. For this solution, we find that the source can only accomodate a moderate amount of beaming, 0.5 $\lesssim b < 1$. Last, we show that the upper limit on the luminosity, L$_X < 2.5 \times 10^{38}$ erg s$^{-1}$ from archival observations, is consistent with a highly-magnetized neutron star being in the propeller phase at that time.