The Rossi X-ray Timing Explorer (RXTE) probes the physics of cosmic X-ray sources by taking their spectra and by making sensitive measurements of their variability over time scales ranging from milliseconds to years. This time behavior is a source of important information about processes and structures in X-ray binary systems, black holes, neutron stars, X-ray pulsars, and X-ray bursters.

Below is list of a few of the types of objects that RXTE observes:

Binary star systems contain two stars that orbit around their common center of mass. Many of the stars in our Galaxy are part of a binary system. A special class of binary stars is X-ray binaries, so-called because they emit X-rays. X-ray binaries are made up of a normal star and a collapsed star (neutron star or black hole). These pairs produce X-rays if the stars are close enough together that material is pulled off the normal star by the gravity of the dense, collapsed star. The X-rays come from the area around the collapsed star where the material that is falling toward it is heated to very high temperatures (over a million degrees!).

An accretion disk is a relatively flat sheet of gas and dust surrounding a newborn star, a black hole, a neutron star, or any massive object growing in size by attracting material, usually from a companion star, as shown above. Matter from an accretion disk is heated as it falls onto the companion, emitting X-rays.

A black hole is a region of space from which neither radiation (light) nor matter can escape. We can most easily detect them in binary systems when we can observe the black hole pulling matter off its companion star into an orbiting disk. As the matter falls close to the event horizon of the black hole, it heats up, releasing X-rays.

A neutron star is the imploded core of a massive star produced by a supernova explosion. Its typical mass is 1.4 times the mass of the Sun and typical radius is about 5 miles. An X-ray pulsar is a rotating neutron star which generates regular pulses of radiation at X-ray wavelengths by accreting matter from a companion star. It may also emit radiation pulses at other wavelengths.

An X-ray burster is an X-ray source in which there may occur very short but very powerful bursts of X-rays. They usually last no longer than 10 seconds, but pack as much energy as the Sun emits in its corona in 3000 years. The bursts are due to sudden burning of hydrogen which has accumulated on the surface of a neutron star. X-ray bursters are usually found in binary systems where the companion star has a Sun-like mass.

Sometimes stars will let out a large amount of energy, which may be visible at more than one wavelength, and then seem to disappear or become quiet, energetically. We call these objects transients because their energy outburst is not permanent, but short-lived. Some transients have outbursts which occur at regular intervals or periodically. Others may outburst unpredictably; some sources have only been observed to burst once and then never again! Transient outbursts are due to a change in rate of material falling onto a neutron star or a black hole. The rate of the infalling material can change if there is an instability in a companion star or an instability in the accretion disk surrounding the neutron star or black hole. A good analogy for transient events is a fireworks display. Your X-ray detector may see a bright flash where there was none before, and then like a burst of fireworks, it fades away.