Relation of the X-Ray Sources Sco X-1 and Cyg X-2 to Old Novae

Abstract

Recent spectroscopic and photometric investigations (Cowley and Crampton 1975; Gottleib et al 1975; Crampton and Cowley 1976) have shown Sco X-1 and Cyg X-2 to be low mass binaries with periods less than a day. Spectroscopically each resembles certain old novae, although in Cyg X-2 the secondary (F) star dominates the spectrum, while for Sco X-1 emission lines from the accretion disk predominate. Both the colors and the rapid flickering in these objects is reminiscent of old novae. Although the masses fall in the usual range for novae and U Gem stars, the periods are about three times the typical values for those stars. In the case of Cyg X-2, one sees spectroscopically that the F star is somewhat evolved, and thus is expanding beyond its Roche lobe as a result of evolution. Although the secondary is not visible in Sco X-1, it is inferred that a similar situation exists. Thus in the low mass X-ray binaries the mass exchange is driven by evolution, resulting in a much higher mass transfer rate than in the case of the novae where the secondary is a lower main sequence star just filling its Roche lobe. It is likely that a further difference is that the X-ray binaries may contain neutron stars rather than white dwarfs as the degenerate member.