Session G07: Fireball Emission from Binary Neutron Star Mergers

The First Unambiguous Electromagnetic Counterpart to a Gravitational-Wave Signal: GRB 170817A and GW170817

On 2017 August 17 at 12:41:06 UTC the Fermi Gamma-ray Burst Monitor (GBM) detected and triggered on the short gamma-ray burst (GRB) 170817A. Two seconds prior LIGO triggered on a binary compact merger candidate, the first from the merging for two neutron stars, associated with the GRB. This is the first unambiguous joint detection of an event in both electromagnetic and gravitational waves, marking the beginning of gravitational-wave multimessenger astronomy. We discuss the GBM observations and analysis of GRB 170817A and the science enabled by the joint GW-GRB detection.   

The Radio Afterglow of GW170817

The spectacular neutron star merger GW170817 was accompanied by radiation across the electromagnetic spectrum. Radio emission was last to be detected, but has proved decisive in establishing the morphology and energetics of merger ejecta, as well as providing constraints on the circum-merger density. The slow continuous rise observed in the radio light curve in the 5 months since the merger requires the presence of a mildly relativistic, wide-angle outflow viewed on axis. This is consistent with the shock breakout of a cocoon of material produced by interaction of a relativistic jet with merger ejecta, which simultaneously can also account for the observed X-ray and gamma-ray emission from GW170817. The radio light curve shows no evidence for the presence of a top-hat jet viewed off-axis and it is therefore unlikely that the jet escaped from the cocoon to successfully produce a SGRB. However this question cannot be completely resolved without additional constraints, for example via ongoing efforts to image the source on mas scales with VLBI.   

Shedding Light on Gravitational Waves

On 2017 August 17, light was observed from the first neutron star merger detected by Advanced LIGO/Virgo, ushering in a new era of combining electromagnetic and gravitational waves to study the Universe. I will describe the efforts by astronomers to discover the optical counterpart, and what was learned from the unique data taken during the first night following the merger. I will then describe the followup work on one of the best studied astronomical transients ever. Finally, I will discuss the remaining mysteries that surround this event, including the origin of the bright blue light seen at early times.