Monitoring the 2010-2015 Hard X-ray/Low-Energy Gamma-Ray Activity of Cygnus X-1 with GBM

Cygnus X-1 is a high-mass X-ray binary with a black hole companion that typically resides in a hard spectral state, where it is extremely bright in hard X-rays and low energy gamma rays and much fainter in the soft X-rays. Since 2008 August, we have used the Gamma-Ray Burst Monitor (GBM) on Fermi to monitor Cyg X-1 in the 10-1000 keV energy range using the Earth occultation technique. Starting in the middle of 2010, Cyg X-1 was observed by GBM to enter a period of increased activity, making several transitions to the soft state, characterized by the typical rise in the soft X-ray flux and decrease in the hard x-ray and low energy gamma-ray flux. From the soft state, Cyg X-1 made several transitions to intermediate states as well as several short transitions back to the hard state. At the end of 2015, Cyg X-1 transitioned back to the canonical hard state, where it has remained ever since. We have generated long-term, broad-band light curves based on daily monitoring of Cyg X-1 over a 9 year period showing the hard-to-soft state transitions, the intermediate states, and the soft-to-hard and failed soft-to-hard state transitions. Spectra are presented of Cyg X-1 in the various states and comparisons made between spectra in the same state. The time evolution of the x-ray hardness ratios is also presented.