We report the detection of coherent pulsations from the ultraluminous X-ray source NGC 7793 P13 .
The \approx 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 ( \gg 5 \sigma ) detection of coherent pulsations demonstrates that the compact object in P13 is a neutron star , and given the observed peak luminosity of \approx 10 ^ { 40 } \text { erg s } ^ { -1 } ( assuming isotropy ) , it is well above the Eddington limit for a 1.4 \text { M } _ { \odot } 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 \approx 1.5 \times 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 .