Constraints on neutrino natal kicks from black-hole binary VFTS 243

TL;DR

This paper uses the black-hole binary VFTS 243 to constrain the natal kick velocity and neutrino-emission asymmetry during black-hole formation, finding very small kicks consistent with theoretical models.

Contribution

It provides the first observational constraints on neutrino-related natal kicks in black-hole formation using VFTS 243 data.

Findings

Natal kick velocity is constrained to be less than 10 km/s.

Neutrino-emission asymmetry is less than 4%.

Black hole experienced a natal kick of about 4 km/s.

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 ($\approx 10\ M_{\odot}$) 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% C.L., the natal kick velocity (mass decrement) is $\lesssim 10$ km/s ($\lesssim 1.0\ M_{\odot}$), with a full probability distribution that peaks when $\approx 0.3\ M_{\odot}$ were ejected, presumably in neutrinos, and the black hole experienced a natal kick of $4$ km/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.