Constraints on Neutrino Natal Kicks from Black-Hole Binary VFTS 243

The recently reported observation of VFTS 243 is the first example of a massive black-hole binary system with negligible binary interaction following black-hole formation. The black-hole mass (≈10M) and near-circular orbit (e≈0.02) of VFTS 243 suggest that the progenitor star experienced complete collapse, with energy-momentum being lost predominantly through neutrinos. VFTS 243 enables us to constrain the natal kick and neutrino-emission asymmetry during black-hole formation. At 68% confidence level, the natal kick velocity (mass decrement) is 10 km/s (1.0M), with a full probability distribution that peaks when ≈0.3M were ejected, presumably in neutrinos, and the black hole experienced a natal kick of 4 km/s. The neutrino-emission asymmetry is 4%, with best fit values of ∼0-0.2%. Such a small neutrino natal kick accompanying black-hole formation is in agreement with theoretical predictions.