What Are X-Ray Binaries?

Present knowledge of X-ray bursters is reviewed. The first X-ray burster was observed to lie in the globular cluster NGC 6624 in Sagittarius, near the galactic center. Other bursts from the same source, occurring at irregular intervals, were then detected and subsequently many other X-ray bursters were discovered, including a rapid burster, producing over 1000 bursts a day, and not exhibiting spectral softening. A model of unstable accretion of matter onto a compact object is proposed to explain the rapid burster. One possible theory of common bursts regards the burst energy as coming from fusion of helium nuclei on the surface of a neutron star. Optical spectra of bursters identified with optical sources have been found to be similar to those of the X-ray binary Scorpius X-1 and cataclysmic variables. Optical emissions have also been observed to coincide with X-ray bursts and further optical observations are needed to determine the nature of these emissions.