Intensity and spectral variability of strong galactic X-ray sources observed by ANS

Abstract

Hard X-ray observations by the ANS are used to analyze the energy spectrum and intensity fluctuations on time scales of seconds to days for 20 luminous galactic sources, of which 11 are galactic-bulge (GX) sources having no obvious identification with source categories. It is found that: (1) most GX sources are best fitted in the energy range from about 1 to 20 keV by exponential spectra with a source temperature (kT) of approximately 5 keV; (2) iron line emission around 6.7 keV is always detected as a persistent spectral feature of at least six and possibly seven of the sources studied; (3) most of the iron-line sources exhibit a direct correlation between kT and intensity (I) in the band from 1.4 to 7.2 keV; (4) no burst source counterparts or candidates show a direct correlation between kT and I, at least two show an inverse correlation, and no iron line emission is detected at a 3-sigma level from burster counterparts during burst-active states. A classification scheme for GX sources is proposed, and several observational tests for this scheme and for possible source models are described.