We report the detection of a 78.1 \pm 0.5 day period in the X-ray lightcurve of the extreme ultraluminous X-ray source NGC 5907 ULX1 ( L _ { X,peak } \sim 5 \times 10 ^ { 40 } erg s ^ { -1 } ) , discovered during an extensive monitoring program with Swift .
These periodic variations are strong , with the observed flux changing by a factor of \sim 3–4 between the peaks and the troughs of the cycle ; our simulations suggest that the observed periodicity is detected comfortably in excess of 3 \sigma significance .
We discuss possible origins for this X-ray period , but conclude that at the current time we can not robustly distinguish between orbital and super-orbital variations .