Probing gamma-ray burst jets with gravitational waves

Gamma-ray bursts (GRBs) are among the most energetic explosions in the Universe. The subset called short GRBs are thought to be produced when two neutron stars merge — precisely the kind of event that gravitational-wave detectors like LIGO are sensitive to. The first joint GW and electromagnetic observation of a binary neutron star merger, GW170817 in August 2017, confirmed this connection directly.

Unlike gravitational waves, which are emitted roughly isotropically by a merging binary, the gamma-ray emission from a short GRB is collimated into a narrow jet along the rotation axis of the remnant. This means an electromagnetic observatory will only detect the GRB if it happens to be pointing close enough to Earth — but the gravitational-wave signal is detectable regardless of viewing angle.

This geometry provides a natural statistical experiment: by comparing how many BNS mergers we detect with gravitational waves against how many are accompanied by a detected GRB, we can constrain the opening angle of the jet without relying on detailed models of the emission mechanism. The wider the jet, the larger the fraction of GW-detected events we would expect to also have an associated GRB counterpart.

The damselfly project develops Bayesian methods for this inference, carefully marginalising over uncertainties in the efficiency with which BNS mergers produce GRBs and other nuisance parameters. As the gravitational-wave detector network accumulates more BNS detections, these constraints will tighten, providing new insight into the physics of relativistic jets and the central engines that drive them.