Walton, D. J.D. J.WaltonFürst, F.F.FürstBACHETTI, MatteoMatteoBACHETTI0000-0002-4576-9337Barret, D.D.BarretBrightman, M.M.BrightmanFabian, A. C.A. C.FabianGehrels, N.N.GehrelsHarrison, F. A.F. A.HarrisonHeida, M.M.HeidaMiddleton, M. J.M. J.MiddletonRana, V.V.RanaRoberts, T. P.T. P.RobertsStern, D.D.SternTao, L.L.TaoWebb, N.N.Webb2020-05-042020-05-0420160004-637Xhttp://hdl.handle.net/20.500.12386/24440We 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}<SUB>{{X,peak</SUB>}}∼ 5× {10}<SUP>40</SUP> erg s<SUP>-1</SUP>), discovered during an extensive monitoring program with Swift. These periodic variations are strong, with the observed flux changing by a factor of ∼3-4 between the peaks and the troughs of the cycle; our simulations suggest that the observed periodicity is detected comfortably in excess of 3σ 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.STAMPAenArticle10.3847/2041-8205/827/1/L132-s2.0-84982295166000381334900013https://iopscience.iop.org/article/10.3847/2041-8205/827/1/L13/meta2016ApJ...827L..13WFIS/05 - ASTRONOMIA E ASTROFISICAScienze Fisiche Settori ERC (ERC) di riferimento::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation