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 (?2.0 M_⊙) 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 ～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 ～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 ～60 000 yr. Furthermore, we predict that the companion star has lost ～10 per cent of its mass as a result of stripping and ablation from the impact of the supernova shell shortly after the explosion.