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

Abstract

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 ($\approx 10M_{\odot }$) and near-circular orbit ($e\approx 0.02$) 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 $\lesssim 10\mathrm{km}/\mathrm{s}$ ($\lesssim 1.0M_{\odot }$), with a full probability distribution that peaks when $\approx 0.3M_{\odot }$ were ejected, presumably in neutrinos, and the black hole experienced a natal kick of $4\mathrm{km}/\mathrm{s}$. The neutrino-emission asymmetry is $\lesssim 4\%$, with best fit values of $\sim 0–0.2\%$. Such a small neutrino natal kick accompanying black-hole formation is in agreement with theoretical predictions.