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

TL;DR

This paper develops a method to determine the spectral states of Cyg X-1 using all-sky monitor data, revealing stable long-term states and short-lived intermediate states with high confidence.

Contribution

It introduces a scheme for consistent state classification across multiple all-sky monitors, enabling long-term and short-term variability analysis of Cyg X-1.

Findings

High agreement (>90%) in state classification across monitors.

Long-term states are stable with over 75-85% probability.

Short-lived intermediate states often last less than three days.

Abstract

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). It also shows the hard and soft states to be stable, with the probability of Cyg X-1 remaining in a given state for at least one week to be larger than 85% for the hard state and larger than 75%, for the soft state. Intermediate states are short lived, with a probability of 50% that the source leaves the intermediate state within three days. A reliable detection of these potentially short-lived events is only possible with monitor data with a time resolution of better than 1 d.