The Variable Superorbital Modulation of Cygnus X-1

We study the superorbital modulation present in the Cygnus X-1 X-ray data, usually attributed to the precession of the accretion disk and relativistic jets. We find a new, strong, 326 ± 2 day period modulation starting in 2005, in Swift BAT and RXTE ASM light curves (LCs). We also investigate Vela 5B ASM and Ariel 5 ASM archival data and confirm the previously reported ~290 day periodic modulation, therefore confirming that the superorbital period is not constant. Finally, we study RXTE ASM LC before 2005 and find that the previously reported ~150 day period is most likely an artifact due to the use of a Fourier-power-based analysis under the assumption that the modulation has a constant period along the whole data sample. Instead, we find strong indications of several discrete changes of the precession period, happening in coincidence with soft and failed state transition episodes. We also find a hint of correlation between the period and the amplitude of the modulation. The detection of gamma rays above 100 GeV with MAGIC in 2006 September happened in coincidence with a maximum of the superorbital modulation. The next maximum will happen between 2008 July 2 and 14, when the observational conditions of Cygnus X-1 with ground-based Cerenkov telescopes, such as MAGIC and VERITAS, are optimal.