DISCOVERY OF COHERENT PULSATIONS FROM THE ULTRALUMINOUS X-RAY SOURCE NGC 7793 P13

We report the detection of coherent pulsations from the ultraluminous X-ray source (ULX) NGC 7793 P13. The approximate to 0.42 s 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, and given the observed peak luminosity of approximate to 10(40) erg s(-1) (assuming isotropy), it is well above the Eddington limit for a 1.4 M circle dot accretor. This makes P13 the second ULX 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 approximate to 1.5 x. 10(12) G, typical of many Galactic 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.