Accretion-to-jet energy conversion efficiency in GW170817

Abstract

Gamma-ray bursts (GRBs) are thought to be produced by short-lived, supercritical accretion onto a newborn compact object. Some process is believed to tap energy from the compact object, or the accretion disc, powering the launch of a relativistic jet. For the first time, we can construct independent estimates of the GRB jet energy and of the mass in the accretion disc in its central engine; this is thanks to gravitational wave observations of the GW170817 binary neutron star merger by the Laser Interferometer Gravitational wave Observatory (LIGO) and Virgo interferometers, as well as a global effort to monitor the afterglow of the associated short gamma-ray burst GRB 170817A on a long-term, high-cadence, multi-wavelength basis. In this work, we estimate the accretion-to-jet energy conversion efficiency in GW170817, that is, the ratio of the jet total energy to the accretion disc rest mass energy, and we compare this quantity with theoretical expectations from the Blandford-Znajek and neutrino-antineutrino annihilation (νν̄) jet-launching mechanisms in binary neutron star mergers. Based on previously published multi-wavelength modelling of the GRB 170817A jet afterglow, we construct the posterior probability density distribution of 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 disc mass, we construct the posterior probability density distribution of the GW170817 remnant disc mass. 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 isη ∼ 10−3, with an uncertainty of slightly less than two orders of magnitude. This low efficiency is in agreement with expectations from the $ \nu\bar\nu $ mechanism, which therefore cannot be excluded by this measurement alone. The low efficiency also agrees with that anticipated for the Blandford-Znajek mechanism, provided that the magnetic field in the disc right after the merger is predominantly toroidal (which is expected as a result of the merger dynamics). This is the first estimate of the accretion-to-jet energy conversion efficiency in a GRB that combines independent estimates of the jet energy and accretion disc mass. Future applications of this method to a larger number of systems will reduce the uncertainties in the efficiency and reveal whether or not it is universal. This, in turn, will provide new insights into the jet-launching conditions in neutron star mergers.