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 ~ 1013 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 four time intervals) without requiring major changes in 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 accommodate a moderate amount of beaming, 0.5 ≲ b < 1. Last, we show that the upper limit on the luminosity, LX < 2.5 × 1038 erg s-1 from archival observations, is consistent with a highly magnetized neutron star being in the propeller phase at that time.