NGC 7793 P13 is an ultraluminous X-ray source harboring an accreting pulsar .
We report on the detection of a \sim 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 \pm 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 \pm 0.13 d . \color black We explored the phase evolution of the X-ray and optical periodicities and propose two solutions .
A superorbital modulation with a period of \sim 2,700–4,700 d probably caused by the precession of a warped accretion disk is necessary \color black to interpret the phase drift of the optical data . \color black We further discuss the implication if this \sim 65 d 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 .