Fast radio bursts are among the strangest mysteries in space science. These pulses last less than five milliseconds but release more energy than the sun does in days or weeks. Since they were first recorded in 2001 (and written about in 2007), scientists have discovered dozens of FRBs. Most are one-off signals, but a few repeat, including one that beats at a regular tempo.
But no one has ever been able to explain what exactly produces FRBs. Before now, only five had been localized to specific regions in space, and they all originated outside our galaxy. When a signal comes from so far away, it’s very hard to find the object responsible for producing it. Most theories have focused on cosmic collisions or neutron stars. And also, well, aliens.
Spoiler alert: it’s not aliens. Two new studies published in Nature today strongly suggest that magnetars—highly magnetized neutron stars—are one source of FRBs. The studies also indicate that these bursts are probably much more common than we imagined.
“I don’t think we can conclude that all fast radio bursts come from magnetars, but for sure models that suggest magnetars as an origin for fast radio bursts are very probable,” says Daniele Michilli, an astrophysicist from McGill University and a coauthor of the first Nature study.
The new findings focus on an FRB detected on April 28 by two telescopes: CHIME (the Canadian Hydrogen Intensity Mapping Experiment, based in British Columbia) and STARE2 (an array of three small radio antennas located throughout California and Utah). The signal, dubbed FRB 200428, released more energy in radio waves in one millisecond than the sun does in 30 seconds.
It’s par for the course for CHIME to find FRBs—it’s found dozens, and in the future the telescope might be able to detect a burst every day. But even though STARE2 was specifically designed to look for FRBs within the galaxy, at lower sensitivities than most other instruments, few expected it to succeed. When it became operational last year, the team predicted a 10% chance it would actually find a signal in the Milky Way.
Then—it happened. “When I first looked at the data for the first time, I froze,” says Christopher Bochenek, a Caltech graduate student in astronomy, who leads the STARE2 project and is the lead author of the second Nature study. “It took me a few minutes to collect myself and make a call to a friend to actually sit down and make sure this thing was actually real.” Between STARE2 and CHIME, this burst was seen by five radio telescopes across North America.
Those observations just happened to coincide with an incredibly bright flash emanating from a highly magnetized neutron star—a magnetar—called SGR J1935+2154, which was located 30,000 light-years from Earth near the center of the Milky Way galaxy.
This magnetar, which is about 40 to 50 times more massive than the sun, produces intense bouts of electromagnetic radiation, including x-rays and gamma rays. Its magnetic fields are so strong that they squish nearby atoms into pencil-like shapes.
Magnetars have always been a suspected source of FRBs, but it’s been difficult for astrophysicists to confirm this, since all other signals came from outside of the Milky Way.
Researchers compared the radio waves of FRB 200428 with x-ray observations made by six space telescopes, as well as other ground-based observatories. Those x-ray emissions pointed to SGR J1935+2154, which flashed 3,000 times brighter than any other magnetar on record.
The CHIME and STARE2 teams deduced that this particular magnetar was responsible for the energetic event that produced not only the bright x-ray emissions but FRB 200428 as well. It’s the first time such a burst has ever been discovered inside the Milky Way, and this FRB emits more energy than any other source of radio waves detected in the galaxy.
FRB 200428 is only a 30th as strong as the weakest extra-galactic FRB on record, and one-thousandth the strength of the average signal. So the fact that STARE2 recorded it after just about a year in operation is a strong indication that these signals are bouncing around the galaxy more frequently than scientists realized.
A counterpoint to these new findings comes from FAST, the Five-hundred-meter Aperture Spherical Telescope, located in southwest China. FAST is the largest single-dish radio telescope in the world. It can’t survey large swaths of the sky, but it can peer narrowly to look for faint signals in places very far away.
FAST studied SGR J1935+2154 for a total of eight hours across four observational sessions from April 16 to 29, according to a third Nature study. And it found no radio waves that coincided with any known x-ray or gamma-ray bursts that happened during that time.
That report doesn’t necessarily nix the magnetar explanation, especially since FAST wasn’t observing during the moment that FRB 200428 was detected. But it does suggest that a magnetar emitting an FRB, if confirmed, is a very rare event, and one that produces radio signals we have yet to fully characterize.
Sandro Mereghetti, an astronomer with the National Institute of Astrophysics in Milan, helped lead the SGR J1935+2154 x-ray detections made by the European Space Agency's INTEGRAL telescope (International Gamma-Ray Astrophysics Laboratory). Though he believes the discovery “strongly favors the class of FRB models based on magnetars,” he points out that “the particular physical processes leading to the observed bursts of radio and hard x-ray emission are not settled yet.” In other words, we don’t know what exactly happens inside a magnetar that would produce FRBs along with associated x rays or gamma rays.
“I would not say that the mystery of FRBs has been solved,” says Mereghetti. “But this is certainly a big step forward that also opens prospects for other similar detections.”