Tracking the Neutron Star ULX NGC 7793 P13 with XMM-Newton to Measure its

Following a series of remarkable discoveries, we now know that someultra-luminous X-ray sources (ULXs) are actually powered by apparentlysuper-Eddington neutron star accretors. So far, five such sources havebeen identified through either their pulsations or the discovery ofcyclotron lines. NGC 7793 P13 stands out among them as the source withthe most easily measurable pulse period and the only ULX pulsar for whichthe companion star was classified. Here we propose two short XMM-Newtonobservations of P13 to follow-up on our successful campaign in AO 16 andAO 17, to track the evolution of the pulse period and to monitor the X-ray spectrum. The timing results will allow us to improve the orbitalephemeris and investigate a possible super-orbital orbital on the orderof 1500 d.