  M82 X-2 is the first pulsating ultraluminous X-ray source (PULX) discovered.
The luminosity of these extreme pulsars, if isotropic, implies an extreme mass
transfer rate. An alternative is to assume a much lower mass transfer rate, but
with an apparent luminosity boosted by geometrical beaming. Only an independent
measurement of the mass transfer rate can help discriminate between these two
scenarios. In this Paper, we follow the orbit of the neutron star for seven
years, measure the decay of the orbit
($\dot{P}_{orb}/{P}_{orb}\approx-8\cdot10^{-6}\mathrm{yr}^{-1}$), and argue
that this orbital decay is driven by extreme mass transfer of more than 150
times the mass transfer limit set by the Eddington luminosity. If this is true,
the mass available to the accretor is more than enough to justify its
luminosity, with no need for beaming. This also strongly favors models where
the accretor is a highly-magnetized neutron star.
