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GW170817 kilonova: what happened next

15 Apr 2018 Hamish Johnston
Biggish bang: artist's impression of a neutron-star merger (Courtesy: NASA)

The physics highlight of 2017 – and our breakthrough of the year – was the first ever multimessenger astronomy observation that involved the detection of gravitational waves. It was the spectacular merging of two neutron stars in a kilonova explosion dubbed GW170817, which has been studied across the electromagnetic spectrum from gamma-rays to radio waves.

The first signals from the kilonova were seen in August 2017, but astronomers are still learning more about the merger and what it created – probably a black hole that is driving an astrophysical jet.

Astronomers are particularly interested in learning more about that jet. Today at the April Meeting of the American Physical Society in Columbus, Ohio, NASA’s Eric Burns gave an update about what the gamma-ray signal tells us about the kilonova.

On  the edge

GW170817 emitted a flash of gamma rays that Burns says resembles a “short gamma-ray burst”. However, it was about 10-100 times dimmer than most other observed short bursts. This could be because we are looking at the edge of the jet or that the jet is not uniform – the jury is still out.

Gravitational waves from the merger were first detected by LIGO–Virgo, and then about 1.7 s later the first gamma rays were seen by the Fermi satellite. Why the delay between the two signals? It is possible that some of that delay is associated with a hypermassive neutron star that existed briefly before collapsing to a black hole. Such a neutron star would give off gravitational waves, but they cannot be detected by LIGO–Virgo.  Another possible contributing factor to the delay is the time that it takes for the jet to form before it starts emitting gamma rays.

The precise nature of the jet was the topic of a talk by Caltech’s Gregg Hallinan, who gave a review of radio observations of GW170817. Radio waves are produced as the jet expands and cools. First, there is an increase in the amount radio waves produced, followed by a drop off that is expected after about 100 days. Hallinan explained that the precise nature of how this drop off occurs provides important clues about the nature of the jet.

Mildly relativistic

Using observations made so far, Hallinan and colleagues reckon that the radio output is consistent with a wide-angle, mildly relativistic outflow that we are viewing along the axis of rotation of the black hole.

The final talk in the session was from Tony Piro of the Carnegie Observatory, who was part of the team that used optical telescopes to pinpoint the location of GW170817. He showed measurements of the visible spectrum of the kilonova, which can be fitted to a black-body curve to give the temperature of the outflow. The first measurement revealed a temperature of 11,000 K, but this dropped quickly to 9500 K in just one hour. Knowing the cooling rate allowed the team to calculate the speed of the outflow, which they reckon is about 30% of the speed of light.

LIGO–Virgo is currently being upgraded and the detectors will come back online later this year for their third observing run. Burns reckons that about one neutron star merger per year could be seen with both gravitational waves and gamma rays, providing more insight into this fascinating phenomenon.

  • To find out more, read our special collection about the detection of gravitational waves by LIGO–Virgo and the emerging field of multimessenger astronomy

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