Direct Statistical Constraints on the Natal Kick Velocity of a Black Hole in an X-Ray Quiet Binary

Abstract

In recent years, a handful of “dark” binaries have been discovered with a nonluminous compact object. Astrometry and radial velocity measurements of the bright companion allow us to measure the post-supernova orbital elements of such a binary. In this paper, we develop a statistical formalism to use such measurements to infer the pre-supernova orbital elements, and the natal kick imparted by the supernova (SN). We apply this formalism to the recent discovery of an X-ray quiet binary with a black hole, VFTS 243, in the Large Magellanic Cloud. Assuming an isotropic, Maxwellian distribution on natal kicks and using broad agnostic mass priors, we find that kick velocity can be constrained to V _k < 72 km s ^−1 at 90% confidence. We find that a Blaauw kick cannot be ruled out, and that at least about 0.6 M _⊙ was lost during the supernova with 90% confidence. The pre-SN orbital separation is found to be robustly constrained to be around 0.3 au.

Keywords

• Black holes
• Bayesian statistics
• Astrophysical black holes
• Binary stars