Circinus X-1: survivor of a highly asymmetric supernova

We have analysed the kinematical parameters of Cir X-1 to constrain the nature of its companion star, the eccentricity of the binary and the pre-supernova parameter space. We argue that the companion is most likely to be a low-mass (less than or similar to 2.0 M-circle dot) unevolved star and that the eccentricity of the orbit is 0.94 +/- 0.04. We have evaluated the dynamical effects of the supernova explosion and we find it must have been asymmetric. On average, we find that a kick of similar to 740 km s(-1) is needed to account for the recently measured radial velocity of +430 km s(-1) (Johnston, Fender & Wu) for this extreme system. The corresponding minimum kick velocity is similar to 500 km s(-1). This is the largest kick needed to explain the motion of any observed binary system. If Cir X-1 is associated with the supernova remnant G321.9-0.3 then we find a limiting minimum age of this remnant of similar to 60 000 yr. Furthermore, we predict that the companion star has lost similar to 10 per cent of its mass as a result of stripping and ablation from the impact of the supernova shell shortly after the explosion.