Some of science's biggest mysteries are in outer space. The identity of dark matter and dark energy involve fundamental questions about how the Universe is constructed. If you instead are interested in mysteries about what the Universe is doing, then fast radio bursts may be at the top of the enigma list.
They are, as their name suggests, fast, lasting for only a handful of milliseconds. And they also involve huge quantities of energy at radio wavelengths, just as promised. But beyond that, we know almost nothing about them, and we have only observed about 35 of them as of last count. Their rarity and transient nature have helped keep them from being better understood.
But this week's edition of Nature includes a collection of 20 new observations, all occurring since the start of 2017. Unfortunately, the new bursts don't tell us much about how they're generated. And, to make matters worse, they suggest that our best bet for figuring it out—the only repeating burst source we know about—is probably unlike all the other sources we're seeing.
It's possible that we have observed fast radio bursts in the past, but their strange behavior—a sudden surge of energy and then nothing—make them seem more like a glitch in the equipment than a real phenomenon. Things are also not helped by the fact that we've now created a vast number of potential radio sources in the vicinity of Earth. But eventually, scientists convinced themselves that what they were looking at was real and represented a large source of energy at a great distance, although it wasn't clear if they were in our own galaxy or not.
Since they come and go so quickly, it has been difficult to localize them to a specific source—they tend to be picked up by broad surveys of the sky, which typically can't give us a precise location. The only exception is a single instance where a source seemed to produce repeated bursts. That object is 3.7 billion light years away, suggesting that we're seeing big events at a great distance.
So what is it? Without knowing where it is, we can't look to image its location to see if there are any hints in what's currently visible. And we can't search archival data to figure out if there was anything unusual in that location before the burst.
This prompted people working on Australia's Square Kilometer Array to use some of the hardware that's already in place to survey the sky to pick up additional sources. This isn't the only survey of its kind, but it has been pretty successful, taking our known instances of fast radio bursts from 36 up to 56. These include both the closest burst yet imaged, as well as the most energetic.
There are also a couple of patterns apparent in the data. One is that the emissions of many of the bursts are biased toward specific radio frequencies. This, however, can either originate at the source or through some interactions between the radio waves and some material they pass through on their way to Earth. So, it doesn't tell us much about what's going on. The total energy of any given burst also seems to have a ceiling, suggesting there's some intrinsic power limit to the process that creates them.
It also looks like there are a lot of them; extrapolating the number found into the whole sky suggests there are over 30 bursts happening every day, and perhaps as many as 45. Because of the way the hardware used in this survey was set up, the same regions of space were scanned multiple times, so if there was a repeating source, it was very likely to be picked up a second time during the survey. None were, a result that is strong enough that we can safely conclude that at least some burst sources never repeat.
That leaves us stuck at one repeating burst source, and here the news isn't that great. Ideally, we'd like the repeater to be as similar as possible to the single-shot radio bursts, because that would be an indication that anything we learn from it would apply to all bursts. But the collected results of several surveys now indicate that the repeating source is relatively weak compared to transient events. Other data have suggested that most fast radio bursts originate in an area with weak or disorganized magnetic fields. The repeater seems to be in an environment with strong magnetic fields.
All of which is suggesting that our best option for studying fast radio bursts may not reflect radio bursts more generally.
Overall, this survey definitely helps us understand the scope of the mystery—we know these things happen often but don't generally happen more than once. Unfortunately, the results don't seem to do too much to solve the mystery itself.
Nature, 2018. DOI: 10.1038/s41586-018-0588-y (About DOIs).
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Ars Technica
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