Long term variability of Cygnus X-1

Abstract

Continuing the observational campaign initiated by our group, we present the long term spectral evolution of the Galactic black hole candidate Cygnus X-1 in the X-rays and at 15 GHz. We present ∼200 pointed observations taken between early 1999 and late 2004 with the Rossi X-ray Timing Explorer and the Ryle radio telescope. The X-ray spectra are remarkably well described by a simple broken power law spectrum with an exponential cutoff. Physically motivated Comptonization models, e.g., by Titarchuk (1994, ApJ, 434, 570, compTT) and by Coppi (1999, in High Energy Processes in Accreting Black Holes, ed. J. Poutanen, & R. Svensson (San Francisco: ASP), ASP Conf. Ser., 161, 375, eqpair), can reproduce this simplicity; however, the success of the phenomenological broken power law models cautions against "overparameterizing" the more physical models. Broken power law models reveal a significant linear correlation between the photon index of the lower energy power law and the hardening of the power law at ∼10 keV. This phenomenological soft/hard power law correlation is partly attributable to correlations of broad band continuum components, rather than being dominated by the weak hardness/reflection fraction correlation present in the Comptonization model. Specifically, the Comptonization models show that the bolometric flux of a soft excess (e.g., disk component) is strongly correlated with the compactness ratio of the Comptonizing medium, with L disk ∝ ( h / s ) −0.19 . Over the course of our campaign, Cyg X-1 transited several times into the soft state, and exhibited a large number of "failed state transitions". The fraction of the time spent in such low radio emission/soft X-ray spectral states has increased from ∼10% in 1996-2000 to ∼34% since early 2000. We find that radio flares typically occur during state transitions and failed state transitions (at h / s ∼ 3), and that there is a strong correlation between the 10-50 keV X-ray flux and the radio luminosity of the source. We demonstrate that rather than there being distinctly separated states, in contrast to the timing properties the spectrum of Cyg X-1 shows variations between extremes of properties, with clear cut examples of spectra at every intermediate point in the observed spectral correlations.

Long term variability of Cygnus X-1

“…We find no such differences; in particular, these dips do not have larger uncertainties in N H and do not show clear trends in their exposure time, χ 2 red , unabsorbed 3-50 keV flux 2 (see also Fig. 5), or the correlation between Γ 1 and Γ 2 known from Wilms et al (2006). The dips cluster around superior conjunction, but individual strong absorption measurements occur as early as φ orb ≈ 0.8 and as late as φ orb ≈ 0.3 (Fig.…”

“…In performing our analysis we used the same approach as we used before (Grinberg et al 2013), i.e., we modeled the data empirically with a broken power law with soft photon index Γ 1 , hard photon index Γ 2 , and a spectral break at ∼10 keV (Wilms et al 2006). This broken power law is modified by a high-energy cutoff, by an Fe Kα-line modeled with a Gaussian at ∼6.4 keV, and by absorption.…”

“…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.…”

“…Ibragimov et al (2005) used 42 Ginga-OSSE and RXTE-OSSE observations of Cyg X-1 taken mainly in the hard state to show orbital variability of absorption by demonstrating an increased equivalent hydrogen column density, N H , around φ orb ∼ 0. In Wilms et al (2006), we analyzed 202 observations from 1999-2004 using RXTE-PCA and, where available, RXTE-HEXTE spectra with models similar to the one presented below and with a time resolution of individual RXTE observations that were typically of several satellite orbits in length, i.e., had exposure times of 5-10 ks. 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.…”