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

Following a series of remarkable discoveries, we now know that some of the mostluminous members of the ultraluminous Xray source (ULX) population are actuallypowered by apparently superEddington neutron star accretors. Three such systemsare currently known\\: M82 X2, NGC 7793 P13, and NGC 5907 ULX1. Here we proposetwo short XMMNewton observations of P13 to followup on our successful campaignin AO 16, to track the evolution of the pulse period, to confirm the orbitalephemeris, and to monitor the Xray spectrum. P13 is the ideal source for thisprogram, as it is the ULX pulsar in which the pulsations are most clearlydetected and the only one for which the companion star is well known, allowingus to fully quantify the orbit and measure the inclination through Xray timing. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]