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

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-disc 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 similar to 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 (P) over dot (over four time intervals) without requiring major changes in (M) over dot, 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 accommodate a moderate amount of beaming, 0.5 less than or similar to b < 1. Last, we show that the upper limit on the luminosity, L-X < 2.5 x 10(38) erg s(-1) from archival observations, is consistent with a highly magnetized neutron star being in the propeller phase at that time.