Nature of Circinus X-1

Abstract

X-ray, radio and optical observations have been used to derive a binary star model for CircinusX-1. Mass transfer between the primary star (Mp ~ 20Mo) and the compact companion star (Me ~ Mo) triggers one or more expanding shock fronts in the vicinity of the compact star. These shocks produce the observed radio emission. Variable optical emission arises both from the changing Roche lobe surface in the highly eccentric system (e ~ 0·8) and from degradation of shock-produced X-ray photons to the optical band by material overlying the expanding shock. The X-ray radiation results from matter in the accretion disc dribbling down on to the surface of the compact star. Mass replenishment at a rate of 5 x 10-8 to 5 X 10-10 Mo per orbit (16·6 day period) occurs near periastron passage. The variation of the X-ray emission arises from absorption in the stellar wind of the primary star. The model predicts an apsidal rotation period for the elliptical orbit of 7-400 yr, an orbital circularization time of ~ 500 yr and a period change of about 0·5 day per 10 yr.