“We have tracked a strange blast of cosmic radio waves back to its home galaxy. This is the second time such a trace has been possible, but the differences from the first one may require us to rethink our ideas of what causes these so-called fast radio bursts (FRBs).
For just 1.3 milliseconds on 24 September, 2018, a barrage of radio waves arrived from space. It was pure luck that all 36 of the antennae that make up the Australian Square Kilometre Array Pathfinder (ASKAP) were pointed in the same direction, towards the area in the sky where the FRB flashed.
That allowed researchers to combine data from all of the antennae and calculate where in the universe it came from – around 13,000 light years from the centre of its home galaxy, which is 4 billion light years away from us. “This is a magnificent technical achievement,” says Shami Chatterjee at Cornell University in New York.
We have spotted 85 FRBs since they were first discovered in 2007, but we have only been able to trace two back to their home galaxies: this one, called FRB 180924, and one discovered in 2012 called FRB 121102. FRB 121102 is one of only two bursts that have ever been caught sending out repeated blasts, which made it far easier to trace.
But FRB 180924 is surprisingly different from 121102, the ASKAP team found. Its host galaxy is 30 times brighter, with light dominated by older stars, whereas 121102 is located in a region of rapid star formation. The repeating FRB was found at the same spot as a constantly glowing radio source, and this one on the outskirts of its galaxy, not near any other radio signals.
The differences are important because, even though FRBs are constantly lighting up all over the universe, we don’t know what causes them. “Whatever exotic thing you can come up with, theorists have come up with a way to make FRBs out of it,” says Chatterjee. Many of those ideas are tailored to FRB 121102, simply because it has been the only one we have had much information about until now.
The plot thickens
It has been a matter of much debate whether all FRBs come from the same type of object or not, particularly because most of them don’t seem to repeat. The fact that these two seem to be so different, and that it is hard to make the explanations for FRB 121102 fit this new source, hint that there may be different kinds of sources for FRBs, says Victoria Kaspi at McGill University in Montreal, Canada.
“We haven’t solved the problem of FRBs with this paper – if anything, the plot thickens,” Kaspi says. “With two objects that seem to be in different classes, maybe that means there are two classes and maybe it means there are 50.”
We will need to find the homes of many more FRBs to be sure – and that will be useful on its own. As the light from an FRB passes through the vast emptiness between galaxies, it picks up signals from anything that might happen to be there.
The ASKAP researchers were able to use these signals to calculate the magnetism of that space – it is very small – and we may eventually be able to use them to learn more about the fundamental nature of our universe and what is in the voids between galaxies, Kaspi says. “If you can detect many of them, you can get snapshots of the universe in all directions.””