Stellar mass black holes

Where can we find black holes as heavy as stars?

An isolated black hole is very difficult to find, but if we have a black hole in an orbit around another star, then we can get an X-ray binary. In such a system the gravitational force from the black hole deforms the other star into the shape of a droplet with the sharp end of the droplet pointing towards the black hole. Matter is streaming out through this sharp end and towards the black hole, around which it forms an accretion disc. The friction between the gas elements in the disc heats up the gas to many million degrees, so that the gas starts to radiate X-rays.

Many of the known X-ray binaries with black holes are not steadily emitting X-rays rather the X-rays are mostly emitted during brief outbursts, that are sometimes called X-ray novae, which occur with time intervals of several years. By studying the spectrum of the X-ray radiation and its rapid time variation during the outbursts it is possible to measure the properties of the gravitational field of the black hole (J. McClintock, 1998, Probing strong gravitational fields in X-ray novae, in S. S. Holt && T. Kallman (eds.) Accretion processes in astrophysical systems: Some like it hot, AIP Conf. Proc. 431, 290). One reason why the black hole system is so faint in X-rays inbetween the outbursts is that the gas in the inner part of the accretion disc is then so thin that it cannot radiate away its heat. Rather the heat disappears together with the gas itself into the black hole (J.-P. Lasota, 1999, ADAFs: Models, observations and problems, Phys. Rep., 311, 247). During the outbursts on the other hand there is more matter flowing inwards through the disc, and then the density is so high that most of the heat of the gas is radiated away before the gas falls into the black hole.