A plain-language summary of A single fast radio burst localized to a massive galaxy at cosmological distance (Bannister et al. 2019, Science; arXiv).
TL;DR: for the first time, we traced a radio flash that never repeated back to the galaxy it came from, using light that set out about 3.7 billion years ago.
The challenge and the solution
Fast radio bursts are flashes of radio waves from deep space that last about a thousandth of a second. Nobody knows for sure what makes them.
By 2019 astronomers had caught dozens of them, but had found the home, i.e. what we call the “host galaxy” of only one. That was possible because astronomers knew it repeats. So you can stare at the same patch of sky until it goes off again. Most bursts are seen once and never again. You get a thousandth of a second, with no warning, to work out where in the sky it came from.
It is a bit like trying to say which house a single camera flash came from, in a city on the other side of the planet, when you didn’t know where to look.
We used a telescope in Western Australia made of 36 radio dishes spread over several kilometres. A burst reaches each dish at a very slightly different moment, and those tiny differences tell you the direction it came from. The trick is that the telescope always holds on to the last few seconds of everything it has recorded, like a dashcam. When a burst is detected, it saves that recording, and we can replay it to pin down the position. The recording was crucial because working out the position from the tiny delays takes quite a bit of computational effort and technical expertise and so it could not be done in real time.
Once the position was known, we could then go and take look with optical telescopes, i.e. the kind that you are probably familiar with and have seen with your local amateur astronomer but just much much more massive. We found a galaxy at that position, and measured its distance. That told us how far away the burst was.
What did we find?
- The burst came from the outskirts of a large galaxy, about 13,000 light-years from its centre.
- That galaxy is big and mostly made of old stars. It looks nothing like the small, busy, star-forming galaxy that hosts the repeating burst. So bursts can come from very different kinds of places.
- On its way here, the burst passed through about as much gas as we expect to lie in the space between galaxies over that distance.
My work for this effort was to help characterize the host galaxy’s light and to estimate galaxy properties such as its distance from us, its mass, and how many stars it is forming currently.
Why does it matter?
More than 95% of all the bursts we have detected to date are one-offs, so showing how one could localize them opened up a whole new area of astronomical research.
Later, when we had a handful of such “localizations”, J. P. Macquart and collaborators showed in 2020 that you could use them as sensitive tools to measure previously invisible gas. Most of my own research since then has built on that idea.
Words you’ll see in the paper
- Localization: pinning down where on the sky a burst came from.
- Redshift: how astronomers measure the distance to a galaxy.
- Dispersion measure: the count of how much gas a burst passed through.