Comparative behaviours of X-ray sources (Cyg X-2, 3U1700-37, A1742-28)

Analysis of the X-ray data from the collimated proportional counters onboard the satellites Copernicus and Ariel V is applied to investigate the differences in the behaviours of the sources Cyg X-2, 3U1700-37 and A1742-28.

Cyg X-2 shows a highly variable X-ray emission; a search for periodicities in its light-curve is carried out on short (order of minutes) and long (several days) time-scales, in the latter case with simultaneous X-ray and optical observations. The spectral characteristics of Cyg X-2 appear to be different from those of the "Sco X-1 type" class, to which the source seems to belong when considering the properties observed in the radio, optical, UV parts of the spectrum. Models for the production of the X-rays by accretion in a binary system are discussed.

The eclipsing binary 3U1700-37 shows high variability in the X-ray emission during its "om" period and different levels in the mean flux over several years. In this case the production of the X-rays is clearly due to accretion from a strong stellar wind; the tail of absorbing material following the compact object around the primary star is interpreted to be the cause of the variable length of the eclipse during observations covering several cycles of the light-curve. The search for periodicity of the order of minutes in the X-ray emission is also described.

The transient X-ray source A1742-28 was discovered in February 1975. The evolution of the source spectral behaviour is studied with the Ariel V data during the period of maximum intensity and with three short Copernicus observations during the subsequent decrease of the X-ray flux. Comparison is made with the other transients observed: A1742-28 emission is also thought to be due to variable mass transfer in a binary system involving a compact object. Its position within 2 arc min from the Galactic Centre is explained by the higher probability of formation of these systems in regions of higher stellar density.