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

D. J. Walton, F. Fuerst, M. Bachetti, D. Barret, M. Brightman, A. C. Fabian, N. Gehrels, F. A. Harrison, M. Heida, M. J. Middleton, V. Rana, T. P. Roberts, D. Stern, L. Tao, N. Webb

Published 2016-07-25Version 1

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.