Long term variability of Cygnus X-1

Abstract

We present a scheme for determining 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). 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 states, which strongly differ in their spectral shape and short-term timing behavior, is only possible when 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., month-to year-long hard-state periods or phases during which numerous transitions occur). It also shows that the hard and soft states are stable, with the probability of Cyg X-1 remaining in a given state for at least one week to be larger than 85% in the hard state and larger than 75% in the soft state. Intermediate states are short lived, with a 50% probability that the source leaves the intermediate state within three days. Reliable detection of these potentially short-lived events is only possible with monitor data that have a time resolution better than 1 d.

Long term variability of Cygnus X-1

“…The total exposure time of our observations is 4.77 Ms which corresponds to ∼10 orbital periods of 5.6 d. The exposure is evenly distributed between the orbital phases. Advances in PCA calibration (Shaposhnikov et al 2012) and a careful consideration of energy-dependent systematic uncertainties in different gain epochs of the PCA instrument Grinberg et al 2013) strengthen our analysis. For a detailed discussion of data treatment, see Grinberg et al (2013).…”

“…Boroson & Vrtilek (2010) classified 102 of these observations as hard states and used them to demonstrate the orbital variability of absorption in the hard state, but did not have enough data taken during softer states. At the same time, higher sensitivity snapshots at different φ orb with Chandra (Miller et al 2005;Hanke et al 2009;Miškovičová et al 2015) and Suzaku Miller et al 2012;Yamada et al 2013) revealed a complex structure of the wind with highly Grinberg et al (2013).…”

“…2). Our total sample consists of 2741 PCU 2 and HEXTE-spectra (Grinberg et al 2013). The total exposure time of our observations is 4.77 Ms which corresponds to ∼10 orbital periods of 5.6 d. The exposure is evenly distributed between the orbital phases.…”

“…The intrinsic variability of Cyg X-1 on timescales of hours (Böck et al 2011) to years (Pottschmidt et al 2003;Wilms et al 2006;Shaposhnikov & Titarchuk 2006;Axelsson et al 2006;Grinberg et al 2013) can be classified in terms of X-ray spectral states that also show distinct timing characteristics on timescales below 1 s and are similar for most black hole binaries (for a review see, e.g., Belloni 2010). In particular, the hard state spectrum above ∼2 keV is dominated by a power law component with photon index Γ ∼ 1.7, while in the soft state the thermal emission from the accretion disk is prominent and the contribution from a steeper power law component is low.…”

“…The 16 years of mainly bi-weekly Cyg X-1 observations (1996-2011) with Rossi X-ray Timing Explorer's (RXTE's) Proportional Counter Array (PCA; Jahoda et al 2006) and High Energy X-ray Timing Experiment (HEXTE; Rothschild et al 1998) allowed us to study the long term spectral and X-ray timing evolution of the source in unprecedented detail in the previous papers of the series (Pottschmidt et al 2003;Gleissner et al 2004b,a;Wilms et al 2006;Grinberg et al 2013Grinberg et al , 2014. In this paper, we use these data to analyze the orbital variability of absorption due to the focussed wind.…”