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 kms-1 is needed to account for the recently measured radial velocity of +430kms-1 (Johnston, Fender & Wu) for this extreme system. The corresponding minimum kick velocity is ~∼ 500 kms-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.