Orbital, Precessional and Flaring Variability in Cygnus X-1

et al. 2006

A&A

147

268

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.

Section: Comptonization: Eqpairsupporting

confidence: 64%

Long term variability of Cygnus X-1

Wilms¹,

Nowak

et al. 2006

A&A

147

268

“…It is persistent, relatively nearby with a radio parallax distance of 1.86 +0.12 −0.11 kpc (Reid et al 2011, consistent with X-ray dust scattering halo estimates of Xiang et al 2011), and bright (above ∼100 mCrab in the 1.5−12 keV band in the hard state), and it often undergoes (failed) state transitions (e.g., Pottschmidt et al 2003), which are thought to be connected to changes in its radio jet (Fender et al 2004Wilms et al 2007). The black hole is in a 5.6 d orbit around its donor star, HDE 226868 (Brocksopp et al 1999b, and references therein). The orbital modulation is also detected in radio (e.g., Pooley et al 1999) and due to modulation of the soft X-ray flux by absorption in the donor's stellar wind in X-rays (Bałucińska-Church et al 2000;Poutanen et al 2008).…”

Section: Introductionsupporting

confidence: 64%

Long term variability of Cygnus X-1

Grinberg

Hell

et al. 2013

A&A

150

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.

“…3. Both the radio flux and the ASM count rate have been rebinned to a resolution corresponding to the orbital period of the HDE 226868/Cyg X-1 system (P orb = 5.599829(16) d, see Brocksopp et al 1999b;LaSala et al 1998). This rebinning removes the effect of the well known orbital variation of the Xray and radio flux (Pooley et al 1999;Brocksopp et al 1999a;Wen et al 2001) such that long term changes become more apparent.…”

Section: Evolution Of the Power Spectrummentioning

confidence: 99%

Long term variability of Cygnus X–1

Wilms

Nowak

et al. 2003

A&A

257

373

Abstract. We present the long term evolution of the timing properties of the black hole candidate Cygnus X-1 in the 0.002-128 Hz frequency range as monitored from 1998 to 2001 with the Rossi X-ray Timing Explorer (RXTE). For most of this period the source was in its hard state. The power spectral density (PSD) is well modeled as the sum of four Lorentzians, which describe distinct broad noise components. Before 1998 July, Cyg X-1 was in a "quiet" hard state characterized primarily by the first three of these broad Lorentzians being dominant. Around 1998 May, this behavior changed: the total fractional rms amplitude decreased, the peak frequencies of the Lorentzians increased, the average time lag slightly increased, and the X-ray spectrum softened. The change in the timing parameters is mainly due to a strong decrease in the amplitude of the third Lorentzian. Since this event, an unusually large number of X-ray flares have been observed, which we classify as "failed state transitions". During these failed state transitions, the X-ray power spectrum changes to that of the intermediate state. Modeling this PSD with the four Lorentzians, we find that the first Lorentzian component is suppressed relative to the second and third Lorentzian during the state transitions. We also confirm our previous conclusion that the frequency-dependent time lags increase significantly in the 3.2-10 Hz band during these transitions. We confirm the interpretation of the flares as failed state transitions with observations from the 2001 January and 2001 October soft states. Both the behavior of the PSD and the X-ray lag suggest that some or all of the Lorentzian components are associated with the accretion disk corona responsible for the hard state spectrum. We discuss the physical interpretation of our results.