A 78 DAY X-RAY PERIOD DETECTED FROM NGC 5907 ULX1 BY SWIFT

We report the detection of a 78.1 +/- 0.5 day period in the X-ray light curve of the extreme ultraluminous X-ray source NGC 5907 ULX1 (L-X,L-peak similar to 5 x 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 similar to 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 cannot robustly distinguish between orbital and super-orbital variations.