J. Casares: Observational evidence for stellar-mass black holes
This is the opening review talk of Symposium 238 on Black Holes: from stars to galaxies asross the range of masses. This will be the last talk I listen to before I have to make my way to the airport, but Ulrike Heiter, a collegue from Uppsala who works on stellar atmospheres, said that she might write a few things from S239 about Convection that starts this afternoon and will go on for the rest of this week.
Now to the talk by Casares: X-ray binaries are believed to be a stellar-mass black hole oriting a normal star. From the study of the radial velocity shift, one can determine the orbital period of the system, and from additional information on the star and its mass and properties (inclination of the system), the mass of the black hole can be calculated. The first example for this was 30 years ago.
The reason why these systems shine in X-rays is that material streams from the "donor star" towards the BH and forms an accretion disk around it that gets hot enough to do that. Depending on the type of the donor star, the accretion disk may even be optically brigher than the star during an active phase.
The basic argument of course is, that when you find an object with a high mass that orbits closely with a star, but you cannot see it and physics tells you that there is no way to have such an object withstanding its own gravitational pull, it must be a black hole. However, the number of dynamically confirmed BHs is still quite low (~20).
Also the lack of pulses and X-ray bursts indicates that there is no hard surface onto which things can bounce.
So how many are there and what is the mass-spectrum? Extrapolating from the known numbers tells that there should be 1000 dormant X-ray Transients (i.e. binaries) in our galaxy (this fits with binary models), but from stellar evolution, there should be 10^8 BHs in the Milky Way, out of which only the tip of the iceberg can be seen as XRTs.
The 15 reliable mass estimates range from 4-15 solar masses with more objects on the low-mass end. The most massive ones seem to lie above the values predicted from stellar evolution (Fryer & Kalogera 1999), but we are talking small-number-statistics here (2 objects).