Circinus X-1: survivor of a highly asymmetric supernova

We have analyzed 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 (∼ 2.0M⊙) 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 is needed to account for the recently measured radial velocity of +430 km s (Johnston, Fender & Wu) for this extreme system. The corresponding minimum kick velocity is ∼500 km s. 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% of its mass as a result of stripping and ablation from the impact of the supernova shell shortly after the explosion.