Astronomers Finally Know What Causes Fast Radio Bursts
Researchers announced yesterday that they’ve solved a question that has been nagging them for over a decade: What exactly produces the weird phenomena known as fast radio bursts? As the name implies, FRBs involve a sudden blast of radio-frequency radiation that lasts just a few microseconds. Astronomers didn’t even know they existed until 2007, but they have since cataloged hundreds of them; some come from sources that repeatedly emit them, while others seem to burst once and go silent.
Obviously, you can produce this sort of sudden surge of energy by destroying something. But the existence of repeating sources suggests that at least some of them are produced by an object that survives the event. That has led to a focus on compact objects, like neutron stars and black holes, with a class of neutron stars called magnetars being viewed very suspiciously.
Those suspicions have now been borne out, as scientists have watched a magnetar in our own galaxy sending out an FRB at the same time it emitted pulses of high-energy gamma rays. This doesn’t answer all our questions, as we’re still not sure how the FRBs are produced or why only some of the gamma-ray outbursts from this magnetar are associated with FRBs. But the confirmation will give us a chance to look more carefully at the extreme physics of magnetars as we try to understand what’s going on.
‘Magnetar’ Is Not the Latest Superhero Film
Magnetars are an extreme form of neutron stars, celestial bodies that are already notable for being extreme. They are the collapsed core of a massive star, so dense that atoms get squeezed out of existence, leaving a swirling mass of neutrons and protons. That mass is roughly equal to the sun’s—but compressed into a sphere with a radius of about 10 kilometers. Neutron stars are best known for powering pulsars, rapidly repeating bursts of radiation driven by the fact that these massive objects can complete a rotation in a handful of milliseconds.
Magnetars are a different type of extreme. They tend not to rotate as quickly but have intense magnetic fields. We don’t know, however, whether those fields are inherited from a very magnetic parent star or generated by superconducting material sloshing around inside the neutron star. Whatever the source, those magnetic fields are about a trillion times stronger than Earth’s magnetic field. That’s strong enough to distort the electron orbitals in atoms, effectively eliminating chemistry for any normal matter that somehow gets close to a magnetar. While the period of high magnetic fields lasts only a few thousand years before the fields dissipate, there are enough neutron stars to keep a regular supply of magnetars around.
Their magnetic fields can power highly energetic events, either by accelerating particles or through magnetic disturbances driven by material shifting within the neutron star. As a result, magnetars have been identified by their semiregular production of high-energy x-rays and low-energy gamma rays, giving them the name “soft gamma-ray repeaters,” or SGR. Several of them have been identified within the Milky Way, including SGR 1935+2154.
In late April of this year, SGR 1935+2154 entered an active phase, sending out a number of pulses of high-energy photons that were picked up by the Swift observatory, in orbit around Earth. That was completely normal. What wasn’t normal is that a number of radio observatories picked up an FRB at precisely the same time.
STARE and a CHIME
The Canadian Hydrogen Intensity Mapping Experiment, or Chime, is a large array of radio antennas that was originally designed for other reasons but has turned out to be great for spotting FRBs, since it can constantly observe a large stripe of the sky. SGR 1935+2154 was at the edge of its field of view, meaning there were some uncertainties in its identity of the source, but the results were clearly consistent with an association between the FRB and the gamma ray output.