EXOSAT observations of the X-ray source in M 15.

Abstract

The results from two long Exosat observations of the X-ray source in the globular cluster M15 (NC 7078), totalling approximately 1 d and separated by 100 d, are reported. Variability is detected on timescales down to 250 s, but no variations attributable to an eclipse were observed, which is surprising in view of the large fluctuations reported by Auriere et al. (1985 and 1986) for the optical candidate AC211. There is no evidence for periodic or quasi-periodic structure in the data. The spectrum is similar in shape to that of many low-mass X-ray binaries, and is best fitted by a two-component model consisting of thermal bremsstrahlung or a power law and a low-temperature black-body with low energy absorption. The absorption is significantly different between the two observations and shows an overall correlation with luminosity, contrary to the Einstein results reported by Hertz and Grindlay (1983). These results are discussed in the context of other low-mass X-ray binaries. The lack of X-ray bursts is attributed to the low metallicity of M15.