Swift Detection of a 65 Day X-Ray Period from the Ultraluminous Pulsar NGC 7793 P13

NGC 7793 P13 is an ultraluminous X-ray source harboring an accreting pulsar. We report on the detection of a similar to 65 day period X-ray modulation with Swift observations in this system. The modulation period found in the X-ray band is P = 65.05 +/- 0.10 days and the profile is asymmetric with a fast rise and a slower decay. On the other hand, the u-band light curve collected by Swift UVOT confirmed an optical modulation with a period of P = 64.24 +/- 0.13 days. We explored the phase evolution of the X-ray and optical periodicities and propose two solutions. A superorbital modulation with a period of similar to 2700-4700 days probably caused by the precession of a warped accretion disk is necessary to interpret the phase drift of the optical data. We further discuss the implication if this similar to 65 day periodicity is caused by the superorbital modulation. Estimated from the relationship between the spin-orbital and orbital-superorbital periods of known disk-fed high-mass X-ray binaries, the orbital period of P13 is roughly estimated as 3-7 days. In this case, an unknown mechanism with a much longer timescale is needed to interpret the phase drift. Further studies on the stability of these two periodicities with a long-term monitoring could help us to probe their physical origins.