WHAT I REALLY WANT TO KNOW IS…
Dr Charles Kilpatrick WKH UVW WR VSRW OLJKW IURP D JUDYLWDWLRQDO ZDYH HYHQW VD\V LW ZLOO KHOS WHOO XV KRZ JROG LV FUHDWHG
In October, astronomers made the dramatic announcement that a ripple in space-time, a gravitational wave, had been matched with an explosive event in a distant galaxy. It’s the first time we’ve ever seen light associated with a gravitational wave event.
I was the first person to set eyes on this remarkable sight, thought to be the merging of two neutron stars in a kilonova. I had spotted it on an image of the galaxy which lies 130 million lightyears away.
I am part of a group called the One-Meter Two-Hemispheres (1M2H) Collaboration. We use two 1m telescopes. One is the Swope at Las Campanas, Chile, and the other is the Nickel telescope, at the Lick Observatory in California. Together, they give us coverage of the entire sky.
The gravitational wave event, labelled SSS17a, was detected on August 17 by two observatories, LIGO in the US and Virgo in Italy. Almost simultaneously, two space telescopes, Fermi and Integral, detected a gamma-ray burst, dubbed GRB 170817A. Along with other astronomers we received a rapid email alert, and quickly swung into action to try to locate it.
LIGO and Virgo pointed us to a region of sky where the wave’s source was thought to be. It was a very large area of around 30 square degrees, equivalent to more than 100 full moons, but we had a strategy for searching for it which involved identifying likely galaxies. We would then photograph those galaxies and look for something new that was not there in archived images.
On high alert
As soon as we got the alert, one of my colleagues put together a list of galaxies at the sort of distance at which the event was thought to have occurred. We then sent that list of targets to our observer on the Swope telescope in Chile. We knew we had to be quick because the patch of sky we needed to check was very close to the Sun and would set a couple of hours after twilight. Our observer went through the galaxies one by one, and I began reducing the data in my office at the University of California Santa Cruz as each image was recorded and sent to me. And in the ninth image, when I looked at a galaxy known as NGC 4993, which is in Hydra, I saw a source that I did not see in the archived picture. Our image of the galaxy was taken almost exactly 11 hours after LIGO detected the gravitational wave, and it took me just 20 minutes to find the visible source. At the time, I was really focused on the process, and it was exciting to be wrapped up in it. But then our team leader Ryan Foley, also of the University of California Santa Cruz, said: ‘Wow. That’s it. That’s the source’. And it very slowly dawned on me what a big deal this was. We had entered a new era in astronomy where we can simultaneously detect gravitational waves and electromagnetic emission from one event. The SSS17a event has been called ‘multi-messenger astronomy’ because it was detected by telescopes observing right across the spectrum, from gammaray and X-ray telescopes in space, through ultraviolet, optical and the infrared, to radio wavelengths. It is important to study because it tells us a lot about where heavy elements in the Universe come from. Neutron stars are incredibly dense. Just a teaspoon of material from one would weigh a billion tonnes on Earth. It is thought that when two neutron stars begin to collide, they throw off neutrons, which form the building blocks of really heavy elements such as gold, platinum, lead and uranium. I’ve heard it suggested that gold several hundred times the mass of the Earth was produced by this one merger. As an optical astronomer, I’m really interested in how often neutron star mergers happen. Hopefully, when we’ve seen a few more of them, we will be able to figure out the rate at which they occur.