Black holes could have atmospheric layers similar to those that surround ordinary stars according to a team of American and Chinese astrophysicists. The team discovered that X-ray emissions from two binary systems - each containing a black hole and a stellar companion - reveal that the black holes are surrounded by three distinct layers similar to those found around the Sun (Science 287 1239).
The team claim to have evidence for: an thin outer layer that is hot; a warm middle layer that is also optically thick; and a “cold” inner layer. The similarity of these layers to that of the corona, chromosphere and photosphere of the Sun suggest that similar physical processes may be at work in both types of object.
In a binary black hole system, the strong gravitational pull of the black hole rips gas and other material from its companion star. This material forms a spiralling accretion disk around the black hole. As the material in the disk approaches the inner region of the black hole, its gravitational energy is released as X-rays and gamma-rays.
Measurements from three satellites — ASCA, the Compton Gamma-ray Observatory and the Rossi X-ray Timing Explorer — indicate that there are three separate components, all with different temperatures, associated with the emissions. Computer modelling by the US-China team suggests that the similarity with the Sun is due to the viscosity of the accretion disc.
