For only the second time, astronomers have picked up a mysterious repeating signal coming from an unknown source in space.
They're called fast radio bursts. And in less than the blink of an eye, they ping into the data collected by radio telescopes out of nowhere, with as much energy as a hundred million Suns.
Most of them only burst once, never to be heard from again. We don't know what they are. We can't track the vast majority.
Only one of these elusive signals had ever been caught repeating, and it was a very big deal, allowing astronomers to trace it to its source for the first time ever - offering a tantalising line to the phenomenon that causes it.
Now, astronomers have found a second.
"Until now, there was only one known repeating FRB," said astrophysicist Ingrid Stairs of the University of British Columbia in Canada.
"Knowing that there is another suggests that there could be more out there. And with more repeaters and more sources available for study, we may be able to understand these cosmic puzzles - where they're from and what causes them."
It's called FRB 180814.J0422+73, and in the space of three weeks beginning in August 2018, it flared six times in repetition, as captured by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope. Further bursts were detected in the following weeks.
In total, CHIME detected 13 new bursts, including the repeating signals.
What makes this even cooler is that these were just test observations for the telescope - it wasn't even operating at full capacity.
The previous repeater, FRB 121102, is famous for its peculiar repetitions.
Because it bursts again and again - with periods of what seem like quiescence in between - astronomers have been able to catch it in the act, and trace it to its source, a galaxy about 3 billion light-years away. And, because the signal has been polarised, or twisted, we know that it passed through an intense magnetic field.
But that's more or less the limit of our knowledge of FRB 121102. We still don't know what causes it, or any of the non-repeating fast radio bursts.
We also don't know what caused FRB 180814.J0422+73, but two things are worth noting.
First, that CHIME is a remarkable piece of equipment that will hopefully do for Northern hemisphere FRB searches what the Australian Square Kilometre Array Pathfinder in Australia did for the Southern hemisphere.
The second is that building a database of these events - and especially repeaters, which can be traced to their home galaxies - will help construct statistics that will narrow down the conditions from which FRBs originate.
FRB 180814.J0422+73, which hails from a galaxy 1.5 billion light-years away, is already providing some new clues as to this great cosmic mystery.
The first is that several of the 13 radio bursts detected by CHIME appeared at a much lower frequency than other fast radio bursts - 400 megahertz, compared to the previous record of 700 megahertz. These are the lowest frequency bursts recorded so far.
This means that, since 400 megahertz is the lower limit of the telescope's capability, the signals may be occurring at lower frequencies still. The 400 megahertz frequency also does rule out some explanations, the researchers said.
"Whatever the source of these radio waves is, it's interesting to see how wide a range of frequencies it can produce. There are some models where intrinsically the source can't produce anything below a certain frequency," said physicist Arun Naidu of McGill University in Canada.
The other big clue is that the 13 signals all show evidence of scattering, or deviation from their original path. From this, astrophysicists are able to make inferences about the environment in which the radio bursts originated.
In this case, the scattering indicates that the bursts originated in an environment with special characteristics.
"That could mean in some sort of dense clump like a supernova remnant," said astronomer Cherry Ng of the University of Toronto. "Or near the central black hole in a galaxy. But it has to be in some special place to give us all the scattering that we see."
It's not quite the same effect as FRB 121102's polarisation, but in both cases, the signal shows characteristics of originating somewhere extraordinary - which makes sense, given the extraordinary nature of the signals.
Further observations will hopefully provide further clues - and so should CHIME, which could detect dozens of signals a day at full capacity. But it's just so exciting how much it has revealed already.
"[We now know] the sources can produce low-frequency radio waves and those low-frequency waves can escape their environment, and are not too scattered to be detected by the time they reach the Earth," said physicist Tom Landecker of the National Research Council of Canada.
"That tells us something about the environments and the sources. We haven't solved the problem, but it's several more pieces in the puzzle."
The team's research has been published in two papers in the journal Nature. They can be found here and here.