  NGC 7793 P13 is an ultraluminous X-ray source harboring an accreting pulsar.
We report on the detection of a ~65 d 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 d 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 d. We explored
the phase evolution of the X-ray and optical periodicities and propose two
solutions. A superorbital modulation with a period of ~2,700-4,700 d 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
~65d 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 d. In this case, an unknown mechanism with a much longer time
scale 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.
