Black hole binaries are systems where an ordinary star and a black hole orbit each other. Gas from the star can be captured by the black hole, and due to the angular momentum, the matter will form an accretion disc around the black hole. As matter moves in through the disc, large amounts of gravitational energy is released. This heats up the disc to such high temperatures that it emits X-rays. By studying this radiation, astronomers try to gain information about the accretion flow. Such studies of black hole binaries by the high energy astrophysics group are presented in two recent papers.

The first studied system is Cygnus X-1, perhaps the most well-known
black hole binary. By investigating how the X-rays change in time, the innermost region of the disc has been studied. The results show that small-scale changes of the disc take longer when the X-ray flux is low. This is interpreted as the disc being truncated at some distance from the black hole. Close to the black hole, the matter instead forms a quasi-spherical flow around the black hole. The results in the paper match those from previous studies of both the spectral energy distribution and analysis of the X-ray variability.

In another study, researchers in the group have studied the spectral
energy distribution of the X-ray emission from Cygnus X-3. The nature
of the accreting object in this system has been subject to much debate. By studying the changes in the X-ray spectrum, the Stockholm University researchers find evidence that the accretor is a black hole, with a mass of around 30 times that of our Sun.

Original articles
Article on Cygnus X-1 by Magnus Axelsson and coworkers
Article on Cygnus X-3 by Linnea Hjalmarsdotter and coworkers Contacts:

Magnus Axelsson +46-8-5537 8515, magnusa@astro.su.se
Linnea Hjalmarsdotter, nea@astro.su.se