X-ray spectral variability of low-mass X-ray binaries

The author reports on a study of the x-ray spectral variability of 37 low-mass x-ray binaries (LMXRB). The sample studied consists of all LMXRB which were detected with the EINSTEIN Monitor Proportional Counter at greater than 10 c s^-1. The spectral variability has been studied through the use of Hardness-Intensity Diagrams (HID). The brighter sources often show complicated HID which contain 'loops', while the HID of the less bright, bursting sources are much simpler. The author speculates on the cause of the 'loops' in the HID, and points out that observations of bursts in the brighter sources may help to determine the cause of the loops. The spectral variations of some of the bursters indicates that Compton scattering mechanisms dominate their x-ray emission. It is shown that the brighter sources may have a higher level of radio emission than the bursting sources, indicating that additional energy generation mechanisms are operating in the brighter sources.