We should be grateful that we’re alive to see the birth of a new type of astronomy.
On 16 October 2017, astronomers from the LIGO-VIRGO collaboration announced that they had detected — for the first time — gravitational waves from the collision of two neutron stars. The detection was made two months earlier, on 17 August so the event was given the name GW170817. But the announcement was of something much, much more exciting than another observation of gravitational waves (tremendously exciting though that is in itself — it is, after all, only the fifth observation of gravitational waves!)
Information from all three LIGO/VIRGO detectors enabled astronomers to localise GW170817 to a patch of sky in the constellation Hydra. However, about 1.74 seconds after detectors were shaken by gravitational waves, the Fermi Space Telescope registered the detection of gamma rays from a gamma-ray burst — the event GRB 170817A, which was in the same patch of sky as the merger. The chances of observing GW170817 and GRB 170817A at the same time and in the same place were tiny — unless, of course, they were the same event!
Astronomers around the world were notified, and about 70 telescopes were trained on Hydra. And so, in the weeks following the merger, astronomers were able to observe the after effects at all the different electromagnetic wavelengths, from gamma rays and X-rays all the way down through visible and radio. This is the most studied event in the history of astrophysics!
It’s difficult to keep up with what this event tells us. To pick three items at random, from the combined observations we now know that:
- colliding neutron stars power short gamma-ray bursts
- elements heavier than iron, such as gold and platinum, form in these collisions
- gravitational waves travel at the speed of light
But it’s the promise of seeing more of these events that is so exciting, because they will enable us to learn so much more about the universe. A merger of two neutron stars is a “standard siren“, which gives us a new method of directly measuring cosmic distances — and thus of measuring the Hubble constant; we can use neutron star mergers to investigate relativity; we can learn much more about the behaviour of matter in these intense conditions; the possibilities are huge. We have now entered the era of multi-messenger astronomy — and it’s going to be wonderful!