Whatever causes fast radio bursts is sitting in an intense magnetic field
The Arecibo Observatory may be suffering along with the rest of Puerto Rico, but some of its data made a big appearance in last week’s edition of Nature. The data comes from the only object of its type we’ve identified yet: a repeating source of fast radio bursts. And, while the new observations don’t definitively tell us what’s creating the bursts, they do suggest that whatever it is, it’s buried in an extremely energetic cloud of material that’s generating some of the most intense magnetic fields we’ve yet found in the Universe. So intense, in fact, that if the source of the magnetic field is a black hole, then it is as massive as 10,000 Suns.
What is that?
A bit over a decade ago, we didn’t even know that fast radio bursts existed. Then a radiotelescope accidentally captured a sudden spike of immense energy that vanished within an instant. That might be dismissed as a hardware glitch, except the observatory eventually caught a few more; over time, dedicated searches revealed that fast radio bursts are a regular, if rare, phenomenon.
The amount of energy produced in a fast radio burst typically comes from a cataclysmic event, one that destroys its source. And indeed, there was no indication of a second burst from any of these sources—but no sign of anything interesting at their location in any other wavelength. The source of fast radio bursts remained a mystery.
Some hope for clearing that mystery up came with the first discovery of an exception: a repeating fast radio burst, called FRB 121102. Over the years, this source has been seen emitting multiple bursts, indicating at least some of these events are not the product of a star-shattering cataclysm. Because there’s every indication that the source is still present, then we might be able to observe it. And the repeated bursts would allow us to get a much better picture of the events themselves.
That’s what the new paper in Nature represents. Over the course of a couple of days in late 2016, researchers used the Arecibo radiotelescope to capture 16 individual bursts. Many of these bursts look quite different from each other. One was a single, sharp peak that lasted only 30 microseconds, which suggests an event that comes from an extremely small area of space (in the neighborhood of 10km, although relativistic effects could change that value). Others were much broader, stretching out to a millisecond, and a few appeared to contain multiple overlapping peaks. So, whatever’s causing this, it’s a fairly variable process.
But the key measurement turned out to be the polarization of the photons. Magnetic fields interact with the polarization of light, rotating it around the axis in which the light is traveling. By measuring this rotation, the authors were able to get a sense of the magnetic environment near the site of the bursts. And it is intense; an accompanying press release calls it “among the most highly magnetized regions ever observed.”
What could be generating these intense magnetic fields? “Such large rotation measures have hitherto been observed only in the vicinities of massive black holes (larger than about 10,000 solar masses),” the researchers note. The bursts appear to be originating near the center of a dwarf galaxy, so this is definitely a possibility. That doesn’t mean the black hole is the source of the bursts—those are typical of the events we see from rapidly rotating neutron stars, and the researchers favor that explanation.
But the black hole could be creating the magnetic fields that are altering the photons produced by the neutron star. Another option is the remnant of a supernova, which is also a possibility, given that the host galaxy is forming stars rapidly.
So we still don’t really know what’s producing fast radio bursts. But these observations tell us something about the environment it’s happening in, and any model for the burst events will have to take them into account. Slowly, we’re narrowing down the possible explanations for these enigmatic events.
Nature, 2017. DOI: 10.1038/nature25149 (About DOIs).
https://arstechnica.com/?p=1244565