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 (≈10 M_⊙) 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 (≲1.0 M_⊙ ), with a full probability distribution that peaks when ≈0.3 M_⊙ 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.