Since the late 60's, there have been many scientific satellites whose mission has been to detect and investigate the X-ray emission of celestial objects. X-rays cannot propagate through the atmosphere and therefore satellites are necessary to detect them.
The all-sky map generated by Uhuru is shown in galactic coordinates (relative to the plane of our Galaxy). A quick look is enough to see that most X-ray sources are on the galactic plane. This must mean that they are inside our Galaxy. A few selected objects are named. Sources which are not at all associated with our Galaxy are marked in green. Galactic sources are in red.
One of the first satellites dedicated to the study of X-rays was Uhuru. It produced a map of the strongest X-ray sources in the sky. Most of the sources were observed to be inside our Galaxy, the Milky Way. Their energy output could be as high as several times 10E38 ergs/s. ``Normal'' stars were sufficiently well known to be sure that they could not produce the X-rays. It was necessary to find a physical explanation for the presence of these point-like high-energy sources. The only possible mechanism for generating the radiation is the gravitational acceleration of particles in the gravitational field of compact objects.
This is the Gamma Ray Observatory (GRO) Compton, one of NASA's satellites. GRO investigates the properties of extremely energetic radiation (hard X-rays and gamma rays). The Bursts And Transient Sources Experiments (BATSE) is an all-sky monitor that watches for any new X-ray sources that turn on. Then the other instruments can be pointed towards the source and analyse the signal.
Compact objects are the result of stellar evolution (what is left after a star finishes its life as a star) and are made of extremely dense matter. There are three kinds of compact objects: white dwarfs, neutron stars and black holes. As the gravitational fields of these objects are enormous, general relativistic corrections are important. For this reason, they are also known as relativistic stars. There are no nuclear reactions balancing the gravitational pull in these objects. All their fuel has been exhausted. The only reason that prevents a white dwarf or a neutron star from collapsing is quantum degeneracy (Pauli's exclusion principle). Therefore, they are also known as degenerate stars. A black hole is a completely collapsed object --- a space-time singularity.
Material falling towards these objects is accelerated to relativistic velocities and then stopped in some kind of collision on or close to the surface of the compact object. Its energy is then liberated in the form of X-rays.
Back to the X-ray Binaries Page.