Stellar Black Hole - X-ray Compact Binary Systems

X-ray Compact Binary Systems

Stellar black holes in close binary systems are observable when matter is transferred from a companion star to the black hole. The energy release in the fall toward the compact star is so large that the matter heats up to temperatures of several hundred million degrees and radiates in X-rays (X-ray astronomy). The black hole therefore is observable in X-rays, whereas the companion star can be observed with optical telescopes. The energy release for black holes and neutron stars are of the same order of magnitude. Black holes and neutron stars are often difficult to distinguish.

However, neutron stars may have additional properties. They show differential rotation, and can have a magnetic field and exhibit localized explosions (thermonuclear bursts). Whenever such properties are observed, the compact object in the binary system is revealed as a neutron star.

The derived masses come from observations of compact X-ray sources (combining X-ray and optical data). All identified neutron stars have a mass below 3 to 5 solar masses. None of the compact systems with a mass above 5 solar masses reveals the properties of a neutron star. The combination of these facts make it more and more likely that the class of compact stars with a mass above 5 solar masses are in fact black holes.

Note that this proof of existence of stellar black holes is not entirely observational but relies on theory: We can think of no other object for these massive compact systems in stellar binaries besides a black hole. A direct proof of the existence of a black hole would be if one actually observes the orbit of a particle (or a cloud of gas) that falls into the black hole.