The Westerbork Synthesis Radio Telescope is scanning the skies
Strange radio signals from space are still baffling astronomers with their odd behaviour. Fast radio bursts (FRBs) are powerful blasts of radio waves that last just a few milliseconds. Some of these bursts have been seen to repeat, flickering on and off many times from the same point in space. They carry a huge amount of energy, but we don’t know what causes them.
The first repeating FRB, called FRB 121102 or R1, was discovered in 2012 and later traced to its host galaxy, a dwarf galaxy about three billion light years away. The second, nicknamed R2, wasn’t found until 2018.
Leon Oostrum at the Netherlands Institute for Radio Astronomy and his colleagues used the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands to watch R1 and R2 for 130 and 300 hours respectively, looking for more bursts that might help characterise them better and find R2’s host galaxy.
While they detected 30 bursts from R1, they didn’t see any from R2. The simplest explanation is that R2 isn’t detectable in the wavelengths at which WSRT observes, which are different from those used by the telescope which discovered it. It would be as if this FRB emits relatively red light, but WSRT can only see blue.
The other possible explanation Oostrum and his colleagues suggest is that R2 could have stopped emitting bursts. However, it is more likely that the telescope can’t detect the FRB’s wavelengths or that any bursts it emitted while Oostrum and his colleagues were observing were just too dim to see, says Jason Hessels, who is also at the Netherlands Institute for Radio Astronomy but wasn’t involved in this work. “Just because you don’t see anything at this time with this telescope doesn’t mean there’s nothing to see,” he says.
Regardless, it shows R1 and R2 are very different from one another. “If the two were similar, we should have seen that second repeater easily, and we didn’t,” says Oostrum. “They could be very different in how bright they are, how often they repeat, and basically any other parameters as well.”
They could also be in very different galaxies, as evidenced by new findings from a separate group led by Hessels. It traced a different repeating FRB called FRB 180916.J0158+65 to its host galaxy, only the fifth time any FRB has been tracked back and only the second repeater to be pinned down in this way.
Its galaxy is completely different from R1’s galaxy. It is a spiral more like our Milky Way instead of an irregularly-shaped dwarf galaxy. Its environment is also far less extreme, making some of the explanations for FRBs that came from analysis of R1 seem less likely.
“We’re in the situation where either a successful theory has to explain that diversity or we have to start thinking seriously about there being multiple different types of sources for FRBs,” says Hessels. If FRBs aren’t all the same but instead result from a variety of different types of events, that could explain why they all seem so different.
FRB 180916.J0158+65 is about six times closer to Earth than R1, so we will be able to observe it in more detail, and the next generation of huge telescopes should help explain FRBs too. “The main goal in the end is to find out what these things are, but for now, the more information we have, the more questions we have,” says Oostrum.