Cygnus X-1, the black hole

You should read first an excellent introduction to identification of Cygnus X-1 as a black hole made by Steven Degennaro.

What is it?

Cygnus X-1 was discovered as an X-ray source 30 years ago (Bowyer et al. 1965). This binary system, distant of 2.5 kpc, consists of the O9.7 Iab type supergiant HDE 226868 (Gies & Bolton 1986) and a compact object orbiting around with a period of 5.6 days. The mass of the unseen companion, significantly larger then 5 solar masses (Dolan & Tapia 1989, Dolan 1992) suggests that it is a black hole. Focused wind accretion (Gies & Bolton 1986) from a primary star being extremely close to filling the Roche lobe drives the powerful source of the X-ray radiation. Cygnus X-1 is one of the brightest X-ray sources in the sky.

Emission variability

X-ray states

X-ray emission of Cygnus X-1 exhibits strong variability at time scales from milliseconds to years. Generally, the X-ray emission falls into one of the two distinct states, named "low" and "high"(Liang & Nolan 1984). Cygnus X-1 spends most of its time (>80%) in the low state, where flux in soft X-ray range (2-20 keV) is lower than in the high state. Ling et al. (1987) have divided the hard X-ray luminosity (45- 140 keV) into three states: gamma1, gamma2, and gamma3. Recent observations (Phlips et al. 1995) suggest that variability does not seem to be between discrete states but rather among continuous range of possible flux values.

Intensity dips

Another temporal feature of Cygnus X-1 are intensity dips (Li & Clark 1974) that preferentially occur near the time of superior conjunction. The duration of the dip may vary from minutes to hours. The flux during the dip decreases in soft X-ray range (E<10 keV), and absorption is generally complex. The partial covering model is consistent with the observational data (Kitamoto et al. 1984). The absorber seems to form a dense blobs of matter, intervening the X-ray source (Pravdo et al. 1980).

The spectrum


The intrinsic X-ray spectrum of Cygnus X-1 can be characterized as a power law of the photon index Gamma ~1.5-1.9 (Liang & Nolan 1984). This kind of the spectrum can be explained by Comptonization of soft photons, presumably coming from the optically thick accretion disk. On top of this continuum one can find an absorption edge at around 7 keV, and a broad feature above 10 keV, called high energy excess (Done et al. 1992). Recent wide-band observations show that the excess has a form of the hump extending from 10 to 200 keV, with the peak at ~30 keV (Gierlinski et al. 1995). Such a hump is considered due to reprocessing and Compton reflection of X-ray photons by an accretion disk (Lightman, White 1988, Magdziarz, Zdziarski 1995).

Iron line

Barr, White, and Page (1985) reported a wide (equivalent width ~120 eV) emission iron K-alpha line at ~6.2 keV. Fabian et al. (1989) attributed this line to the fluorescence of the inner part of the accretion disk, at a few Schwartzschild radii. Such an emission should exhibit a characteristic, double-wing profile. More recent work (Ebisawa et al. 1995) show that the observed feature is consistent rather with the narrow gaussian line (1 sigma < 0.2 keV), suggesting that it can come from the outer part of the disk. The weakness of the line (equivalent width ~20 eV), inconsistent with theoretical predictions (George, Fabian 1991), is not understand well.


At higher energies (E > 100 keV) observed flux falls down exponentially. In most cases gamma-rays from Cygnus X-1 can be approximated by a power law with exponential cutoff. Haardt et al. (1993) showed, using Monte Carlo methods, that inverse Compton continuum from the optically thin (tau=0.3) and hot (kT=150 keV) plasma is consistent with SIGMA and OSSE observations.

Something around 1 MeV

Several authors have reported a hardening of the spectrum in the region around 1 MeV (Ling et al. 1987, Bassani et al. 1989), though many other observations have found no evidence for such emission (e.g. McConnel et al. 1994). It suggests that this spectral feature must be transient. Liang and Dermer (1988) proposed explanation of the 1 MeV bump in context of the hot (kT ~ 400 keV) electron-positron plasma in the inner region of the accretion disk.

Cygnus X-1 and Active Galactic Nuclei

Many of the spectral features mentioned above have been also found in the spectra of Active Galactic Nuclei (see e.g. Mushotzky, Done, Pounds 1993, Zdziarski 1995). Both in the case of the Cygnus X-1 and an AGN an X-ray emission seems to origin from the accretion onto black hole. We observe a power-law intrinsic spectrum, a disk reflection feature and an iron line. We are convinced that research on this "miniature AGN" can turn out a significant contribution to the understanding of AGNs.