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 .