Astronomers were already struggling to explain Fast Radio Bursts (FRBs) when they discovered the even more puzzling phenomenon of a repeating FRB. Now things have become even more confusing in the form of another repeating FRB, described by its discoverers as “younger and weirder” than the first. Strange as the new discovery is, it could hold the key to explaining at least one class of FRBs.
The new repeating object, named FRB 190520B, is located on the outskirts of a dwarf galaxy almost 3 billion light-years away. Dispersion of the radio signal (where low-frequency waves are delayed by passing through free electrons compared to high-frequency counterparts) reveals the highest electron density of any FRB host galaxy so far found.
After originally being discovered with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), the journal Nature now reports the detection of 87 follow-up bursts using the Very Large Array. Just how often the bursts occur is hard to say, with five observed at a frequency of 3 GHz in a 16-hour observation period, but fewer at other radio frequencies.
As the name suggests, FRBs involve very powerful radio wave emissions lasting just a few milliseconds, in which time as much energy can be released as the Sun does in days. So far, only 5 percent have been found to repeat – but it's likely others also do, so either on timelines so long we can't see them or simply when we haven't been looking. There is speculation all FRBs repeat, but we're obviously a long way from being able to confirm that.
One thing greatly hindering our capacity to understand FRBs' causes is that while we can identify the host galaxy, we usually can't find the actual source. As a rule, there's no object visible at any frequency we can tie the FRB to.
FRB 190520B is a very important exception – the bursts have been confirmed to be associated with a persistent radio source (PRS). This is only the second time this has happened – the first time was with FRB 121102A, the first repeating FRB detected.
In theory, astronomers might be able to identify the nature of the source of the PRS, and therefore the FRB. We're not there yet, but the authors do think FRB 190520B's extreme electron dispersion is indicative of the FRB being located in a complex plasma environment that at least partially resembles a superluminous supernova. This could be a sign the FRB has recently “switched on” or started emitting.
FRB 190520B and FRB 121102A share more in common than just being the only two FRBs known to be associated with a PRS. Both are particularly active, even among repeating FRBs, and both are coming from electron-rich environments – in 190520B's case, some ten times higher than normal. On each count, however, FRB 190520B is more extreme, explaining why the authors consider it the weirder of the siblings.
"Now we have two like this, and that brings up some important questions," said co-author Dr Casey Law of Caltech in a statement.
"Are those that repeat different from those that don't? What about the persistent radio emission – is that common?" added West Virginia University graduate student Kshitij Aggarwal.
FRBs have been proposed to come either from recent supernova remnants or from magnetars. It's possible FRB 190520B and FRB 121102A – and most other known FRBs – form two different populations with this pair representing newborn supernova debris, while most others are magnetars.
"We further postulate that FRB 121102A and FRB 190520B represent the initial stage of an evolving FRB population. A coherent picture of the origin and evolution of FRBs is likely to emerge in just a few years," said Dr Li Di of the University of Chinese Academy of Sciences in another statement.
It was all so much easier with the FRB-like signals that turned out to be from impatient astronomers opening the microwave oven in the tea room before it was finished.