A 78 Day X-ray Period Detected from NGC 5907 ULX1 by Swift
  We report the detection of a $78.1\pm0.5$ day period in the X-ray lightcurve of the extreme ultraluminous X-ray source NGC 5907 ULX1 ($L_{\rm{X,peak}}\sim5\times10^{40}$ 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. 
D. J. Walton, F. Fuerst, M. Bachetti, D. Barret, M. Brightman, A. C.   Fabian, N. Gehrels, F. A. Harrison, M. Heida, M. J. M...
astro-ph.HE astro-ph.GA