For a fraction of a second in late April, a hyper-magnetized star in the Milky Way suddenly blasted out radio energy. Now scientists say that this sudden, strange blip could help to explain one of astronomy’s biggest puzzles: what powers the hundreds of other mysterious fast radio bursts (FRBs) that have been spotted much farther away in the Universe.
The star, known as SGR 1935+2154, is a magnetar — a dense, spinning ember left behind after a supernova and wrapped in intense magnetic fields. Many astronomers think that fast radio bursts — brief but powerful cosmic flashes that flare for just milliseconds — come from magnetars, but haven’t been able to show the link.
“I wouldn’t say it’s the nail in the coffin that we’ve figured out that fast radio bursts come from magnetars,” says Emily Petroff, an astronomer at the University of Amsterdam in the Netherlands. “But it’s by far the most promising piece of evidence that we’ve found.”
Preliminary papers describing the burst, which is the first to be detected in the Milky Way, have flooded the arXiv preprint server in recent days.
Until now, the closest known fast radio burst happened around 150 million parsecs (490 million light years) from Earth. This magnetar is in our Galaxy just 10,000 parsecs away, making it close enough for astronomers to have a great view as it sizzles with activity. “Here is something that gets close to the insane intensity of cosmic FRBs, but that is happening not so far away,” says Sarah Burke Spolaor, an astronomer at West Virginia University in Morgantown. “It’s a fantastic opportunity to learn about at least one of the sources that could be causing FRBs.”
The show began on 27 April, when satellites including NASA’s Neil Gehrels Swift Observatory spotted γ-rays streaming from SGR 1935+2154. The star is one of about 30 known magnetars in the Milky Way; these occasionally go through spurts of activity during which they emit radiation at different wavelengths. The next day, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope in Penticton, Canada, detected a huge radio flash occurring to the side of its field of view — from the place in the sky where the magnetar lay1.
The CHIME team had been hoping to pick up radio emission from SGR 1935+2154. But they were expecting faint radio pulses. Instead, “we got something much more exciting”, says Paul Scholz, an astronomer at the University of Toronto who led the analysis.
A second research team got even luckier by catching the intense burst full-on. The STARE2 radio telescope is made of low-tech antennas — each consists of a metal pipe with two cake tins attached — at two locations in California and one in Utah. STARE2 has been observing the sky since last year, hoping to catch something resembling a fast radio burst in the Milky Way. On 28 April, it did exactly that, detecting the same radio pulse that CHIME saw2. “I was so excited that it took me a little bit of time to open up the data and inspect it, to make sure it was real,” says Chris Bochenek, a graduate student at the California Institute of Technology (Caltech) in Pasadena who works on STARE2. “Chris messaged us on Slack, and fairly unrepeatable things were said,” says Vikram Ravi, an astronomer at Caltech and Bochenek’s co-adviser.
The radio flash is by far the brightest ever seen from a magnetar in the Milky Way, and could offer clues to what causes fast radio bursts seen elsewhere in the Universe.
Because magnetars are spinning quickly and have powerful magnetic fields, they have huge reservoirs of energy that can produce outbursts. One idea about the source of these outbursts is that something happening inside the magnetar — such as a ‘starquake’, analogous to an earthquake — could crack its surface and release energy. Another possibility is that the highly magnetized environment around the magnetar somehow produces the burst.
Astronomers might be able to narrow down these possibilities by studying both the radio burst from SGR 1935+2154 and bursts in other wavelengths of light that happened simultaneously, says Laura Spitler, an astronomer at the Max Planck Institute for Radioastronomy in Bonn, Germany. Several satellites detected X-ray bursts from the magnetar at around the same time as the radio emission3,4,5. It is the first time astronomers have detected these signals in other wavelengths; seeing them was possible only because the magnetar is so close to Earth.
But some mysteries remain. For one thing, the 28 April burst was about 1,000 times less energetic than fast radio bursts seen in distant galaxies. And some distant bursts repeat at intervals that can’t easily be explained as coming from a magnetar. Perhaps some, but not all, fast radio bursts come from magnetars, says Petroff.
Astronomers still want to collect as many examples of fast radio bursts as they can, both near and far away. “Each serves as a kind of backlight shining through all the material between us and the source,” says Jason Hessels, an astronomer at the University of Amsterdam. Scientists have recently started to use that information to map the distribution of matter in the Universe6.
“There’s an exciting future to the field,” says Hessels, “even if this is more or less the answer to where the bursts are coming from.”
Nature 582, 322-323 (2020)