Neutron star merger as it happened

All around the world: observatories that saw GW170817 (Courtesy: LIGO-Virgo)

By Hamish Johnston

At 12:41:20 UTC on 17 August, the Fermi Gamma-ray Space Telescope sent a notice to the astronomy community saying that it had detected a gamma-ray burst. Normally such an event wouldn’t raise much fuss – but this time things would be very different.

Back on Earth, scientists working on the LIGO-Virgo gravitational wave detectors were busy analysing a signal that had arrived about 2 s before the gamma-ray burst and looked very much like the merger of two neutron stars. Such a cataclysmic event is expected to give off copious amounts of electromagnetic radiation including an initial burst of gamma rays.

LIGO-Virgo sent its first notice at 13:21:42 saying that it had spotted a neutron-star merger and by midnight the team was already telling other astronomers where to point their telescopes to capture light from the event.

At 01:05:23 on 18 August, the Swope Telescope at Cerro Las Campanas in Chile was the first to report seeing an optical signal. As well as detecting the first visible light from a neutron-star merger, Swope also had the first-ever view of a “kilonova”. Hitherto hypothetical, this is a huge explosion that astrophysicists now know produces heavy elements including most of the gold in the universe.

The floodgates then opened and sightings came fast and furious from dozens of other optical, infrared, radio and X-ray telescopes. The era of multimessenger astronomy was well underway less than 24 h after the first observations by LIGO-Virgo and Fermi.

You can read a minute-by-minute, telescope-by-telescope account of this amazing observation in “Multi-messenger observations of a binary neutron star merger”, which is free to read in The Astrophysical Journal Letters.