Accretion-to-jet energy conversion efficiency in GW170817

TL;DR

This paper estimates the efficiency of converting accretion disk energy into jet energy in GW170817, finding a low efficiency consistent with neutrino annihilation and Blandford-Znajek mechanisms, considering uncertainties and merger dynamics.

Contribution

It introduces a new numerical-relativity-informed method to estimate accretion-to-jet efficiency in GW170817, integrating multi-wavelength afterglow data and disk mass modeling.

Findings

Accretion-to-jet efficiency is approximately 10^{-3}.

Efficiency aligns with neutrino annihilation and Blandford-Znajek mechanisms.

Uncertainties are carefully quantified.

Abstract

Based on previously published multi-wavelength modelling of the GRB 170817A jet afterglow, that includes information from the VLBI centroid motion, we construct the posterior probability density distribution on the total energy in the bipolar jets launched by the GW170817 merger remnant. By applying a new numerical-relativity-informed fitting formula for the accretion disk mass, we construct the posterior probability density distribution of the GW170817 remnant disk mass. By combining the two, we estimate the accretion-to-jet energy conversion efficiency in this system, carefully accounting for uncertainties. The accretion-to-jet energy conversion efficiency in GW170817 is $\eta\sim 10^{-3}$ with an uncertainty of slightly less than two orders of magnitude. This low efficiency is in good agreement with expectations from the $\nu\bar\nu$ mechanism, which therefore cannot be excluded by this measurement alone. Such an efficiency also agrees with that anticipated for the Blandford-Znajek mechanism, provided that the magnetic field in the disk right after the merger is predominantly toroidal (which is expected as a result of the merger dynamics).