A 78 Day X-ray Period Detected from NGC 5907 ULX1 by Swift

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 ($L_{\mathrm{X},\mathrm{p}\mathrm{e}\mathrm{a}\mathrm{k}}\sim 5\times {10}^{40}$ 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 cannot robustly distinguish between orbital and super-orbital variations.