  We report the detection of coherent pulsations from the ultraluminous X-ray
source NGC 7793 P13. The ~0.42s nearly sinusoidal pulsations were initially
discovered in broadband X-ray observations using XMM-Newton and NuSTAR taken in
2016. We subsequently also found pulsations in archival XMM-Newton data taken
in 2013 and 2014. The significant (>>5 sigma) detection of coherent pulsations
demonstrates that the compact object in P13 is a neutron star with an observed
peak luminosity of ~1e40 erg/s (assuming isotropy), well above the Eddington
limit for a 1.4 M_sun accretor. This makes P13 the second ultraluminous X-ray
source known to be powered by an accreting neutron star. The pulse period
varies between epochs, with a slow but persistent spin up over the 2013-2016
period. This spin-up indicates a magnetic field of B ~ 1.5e12 G, typical of
many accreting pulsars. The most likely explanation for the extreme luminosity
is a high degree of beaming, however this is difficult to reconcile with the
sinusoidal pulse profile.
