Long term variability of Cygnus X-1 V. State definitions with all sky monitors

Understand

We present a scheme to determine the spectral state of the canonical black hole Cyg X-1 using data from previous and current X-ray all sky monitors (RXTE-ASM, Swift-BAT, MAXI, and Fermi-GBM).

• State determinations of the hard/intermediate and soft state agree to better than 10% between different monitors, facilitating the determination of the state and its context for any observation of the source, potentially over the lifetimes of different individual monitors.
• A separation of the hard and the intermediate state, which strongly differ in their spectral shape and short-term timing behavior, is only possible when monitor data in the soft X-rays (<5 keV) are available.
• A statistical analysis of the states confirms the different activity patterns of the source (e.g., months to years long hard state periods or phases during which numerous transitions occurs).