Long-term variability of galactic X-ray sources

Abstract

THE long-term (days to months) variability of 32 persistent galactic X-ray sources brighter than Cas A in the energy range 3–10 keV is discussed here, using data from the MIT experiment on the OSO 7 satellite¹. For each X-ray source, we compiled a list of counting rates for all observations made during the first 600 d of operation of the experiment² (typically six or seven observations for each source). The results in these lists show the well-known high variability of the massive early-type X-ray binaries. Of special interest, however, are the variability characteristics of the sources associated with old populations. These include several near the galactic centre which are generally referred to as ‘bulge’ sources, and several which are known to lie in globular clusters. They share certain similar X-ray characteristics, namely, spectra which are markedly softer than those of massive binaries and an apparent absence of pulsations and eclipses^3–6. Because of their similarities, it has been suggested that they are all systems of the same kind, such as neutron stars in low mass binary systems^7,8, or the products of core collapse in globular clusters⁹. However, our results show that the long-term variability of the bulge sources is generally much less than that of the sources which lie in globular clusters. This suggests that there is a substantial difference between the bulge and globular cluster sources either in their nature or, at least, in certain critical properties of their components such as the masses of the nuclear-burning companions which may supply the material for accretion by compact objects in binary systems.