Long-term pulse period evolution of the ultra-luminous X-ray pulsar NGC 7793 P13

The ultra-luminous X-ray (ULXs) pulsar NGC 7793 P13 is one of only a handful confirmed neutron stars in an 19.5 ULX and can reach luminosities above 1040 erg/s, more than 20x its Eddington luminosity. It shows very strong pulsations with a pulse period around 415ms. We’ve been monitoring the pulsations over the last years and 21 used them to determine the orbital period to be ~65d. In addition, P13 shows a very strong long-term flux period in the X-rays and the optical/UV, with periods close to but significantly different to the orbital period. Understanding the pulse period evolution and its connection to the overall flux and flux variability will help us gain important insights into super Eddington accretion flows.