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
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.
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.
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).
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.
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