The variable super-orbital modulation of Cygnus X-1

TL;DR

This study reveals that Cygnus X-1's super-orbital modulation is variable, with a newly identified 326-day period, and shows that previous constant-period assumptions were artifacts, highlighting complex precession behavior.

Contribution

It uncovers the non-constant nature of Cygnus X-1's super-orbital period and identifies multiple discrete changes linked to state transitions, challenging prior assumptions of a stable period.

Findings

Discovery of a new 326-day super-orbital period starting in 2005.

Confirmation of a previously reported ~290-day period, showing period variability.

Evidence that the ~150-day period was an artifact of analysis assumptions.

Abstract

We study the super-orbital 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 d period modulation starting in 2005, in Swift/BAT and RXTE/ASM light curves (LCs). We also investigate Vela5B/ASM and ArielV/ASM archival data and confirm the previously reported ~290 d periodic modulation, and therefore confirming that the super-orbital period is not constant. Finally, we study RXTE/ASM LC before 2005 and find that the previously reported ~150 d 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 September 2006 happened in coincidence with a maximum of the super-orbital modulation. The next maximum will happen between 2 and 14 of July 2008, when the observational conditions of Cygnus X-1 with ground-based Cherenkov telescopes, such as MAGIC and VERITAS, are optimal.