Black hole binaries

One of the best ways to get some information about black holes is if they are part of a binary system, in which gas streams from the companion (in)to the black hole. Such binary systems form a gas disc around the black hole (an accretion disc) which become so hot that it emits X-rays which we can detect with special satellites.

One of the fundamental questions about black holes is how they form. We believe that they form when the compact core of a very massive star collapses. However, how massive the star has to be and how the black hole is formed (i.e. does the whole star collapse, or is part of it ejected into space in a supernova explosion) is not clear yet.

We inferred some information about these questions by looking at the space velocity of some black hole binaries. From the velocities we could determine limits on the amount of mass that must have been ejected.

If the space velocity of the binaries cannot be measure directly, an indirect measurement can be obtained from the height of the binary above the Galactic plane, assuming that the system originated in the plane. For the Jan van Paradijs memorial symposium we wrote a small paper on this.

Later, we collected the most recent distance and position measurements of black hole binaries and showed that the situation had changed compared to an earlier analysis by White & van Paradijs (1996). There is no longer evidence that the black hole binaries are on average closer to the Galactic plane than the neutron star binaries (for details see Jonker & Nelemans, 2004).