A Study of Four X-ray Sources with Properties Similar to Sco X-1

Abstract

We present observational evidence, gathered with the Copernicus and Ariel V satellites, that 3U 0614 + 09, 3U 1702 – 36 (Sco X-2), 3U 1728 – 16 (GX9 + 9) and 3U 1758 – 25 (GX5 – 1) exhibit X-ray behaviour which establishes them in a class with Sco X-I. The characteristic feature of these X-ray sources is irregular flux changes on timescales typically of the order of an hour, which are correlated with the spectral slope in such a way that the spectrum becomes harder as the flux increases. (This is the opposite of the effect expected if the flux changes were produced by photoelectric absorption.) In addition, 3U 1758 – 25 also shows longer term spectral changes with a timescale of days. Three of the five sources discussed (3U 0614 + 09, Sco X-1 and 3U 1728 – 16) have been identified with ultraviolet bright stars. We point out that the ratio of the X-ray to optical luminosity of these three sources is approximately the same, thereby adding weight to the suggestion that the emissions arise in intrinsically similar systems. Some physical consequences are discussed.