# SWIFT Detection of a 65-Day X-ray Period from the Ultraluminous Pulsar NGC 7793 P13

## TL;DR

This study reports the detection of a ~65-day X-ray and optical periodicity in the ultraluminous pulsar NGC 7793 P13, suggesting complex superorbital and orbital modulations possibly caused by disk precession and other mechanisms.

## Contribution

First detection of a ~65-day X-ray and optical periodicity in NGC 7793 P13, with analysis of phase evolution indicating superorbital and orbital modulations.

## Key findings

- X-ray modulation period of 65.05 days with asymmetric profile
- Optical modulation period of 64.24 days confirmed by UVOT
- Possible superorbital modulation with 2,700-4,700 days affecting phase drift

## Abstract

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.