A new mystery of a well-known black-hole binary revealed by INTEGRAL
The Galactic binary system Cygnus X-1, discovered in 1964, hosts the
first widely accepted black hole. Being one of the best studied object
of its class, it still remains puzzling in some respects. A recent
analysis of the long-term observations with the INTEGRAL satellite
uncovered one more puzzle, related to the accreting system geometry.
INTEGRAL observed Cygnus X-1 many times in the 2002-2017 period,
collecting the largest hard X-ray (20-100 keV) data set for this source
(almost 8000 1-2 hour long observations). The emission from this
black-hole system in this energy band is believed to be dominated by the
inverse Comptonization process, where the soft (from infrared to soft
X-ray) photons from the accretion disk are upscattered to higher
energies by the hot electrons from the plasma region near the black hole
(or above the disk). Comptonization models predict that, if these
electrons are mostly thermal, the slope of the hard X-ray spectra is
predominantly regulated by the geometry of the disk-plasma system, with
changing sizes of the disk and plasma regions.
Analysis of the INTEGRAL data shows that there are several distinct
regions seen in the 2D density diagram with the hard X-ray flux plotted
against the hard X-ray spectral slope (photon index). Six regions were
identified (see Figure) and confirmed with an additional study of
several other parameters of the system. Such result is surprising,
because a kind of a continuous transition between the two extreme
spectral states (pure soft, disk-dominated and pure hard,
plasma-dominated) was expected, as it is observed in the soft X-ray
band. The uncovering of several distinct accretion modes in Cygnus X-1
(artist impression as inset) was possible, because the hard X-rays are
emitted exclusively by the plasma and INTEGRAL provided the first huge
data set of high quality in this energy band.
No clear theoretical explanation of the distinct modes exist. One may
invoke three main possibilities to explain the observed behaviour:
distinct accretion rates (e.g., regulated by states of the donor star),
specific structures of the accretion disk (spirals, warps, resonances)
or specific geometries of the plasma region (possibly related to the jet
activity). Future INTEGRAL observations of Cygnus X-1, together with a
more advanced spectral and timing analysis should help to solve the puzzle.
