Exotic Fast Radio Burst Goes Up in Smoke

What was believed to be a unique and exciting fast radio burst in a far-away galaxy turns out to be a rather mundane pulsar in our own Milky Way. The 2022 Nature paper describing the discovery of the burst and its potential implications will need to be retracted. “It’s not something you’d like to happen,” says team member Vicky Kaspi (McGill University, Canada).

Fast radio bursts (FRBs) are millisecond-long explosive events that are probably produced by magnetars — strongly magnetized neutron stars — in distant galaxies. Thousands have been observed so far, but much about them remains unknown. (Read what we know about fast radio bursts in the September 2022 issue of Sky & Telescope.)

Little wonder that astronomers were excited about the new event, dubbed FRB 20191221A, detected on December 21, 2019, by the CHIME radio telescope in British Columbia. This burst had an unusual long duration of about three seconds and showed nine regular sub-bursts with a periodicity of 217 milliseconds.

“Such short periodicity provides strong evidence for a neutron-star origin of the event”, the authors wrote in their discovery paper. The radio pulses would result from the neutron star’s rapid rotation, just like in the case of pulsars.

The team didn’t initially think this was a regular pulsar, though. The radio bursts were smeared out so that the lower frequencies arrived later than the higher ones — an effect, known as dispersion, that indicates intervening matter. Material in the Milky Way alone couldn’t explain the signal’s strong dispersion, given that the burst originated pretty far from the central plane of the Milky Way.

Instead, the team concluded, the FRB must lie far beyond the Milky Way, at a distance of about 3 billion light-years, with tenuous intergalactic plasma producing most of the observed dispersion. For us to see a pulsar at this distance, it would have to be incredibly luminous – more comparable to a fast radio burst than to regular pulsars. The team concluded at the time that this was the first FRB to pulse in a regular way.

Back then, some astronomers wondered why this “heartbeat” behavior had never been seen in other FRBs. Others thought that a neutron star would need to rotate much faster than once every 217 milliseconds to produce such powerful bursts. “It was certainly puzzling,” says team member Kendrick Smith (Perimeter Institute, Canada), “but it looked so solid.”

New Analysis, Retracted Claim

Now, FRB20191221A has gone up in smoke. At this week’s FRB2025 meeting in Montreal, Daniele Michilli (now at the Laboratory of Astrophysics in Marseille, France), who led the original discovery team, told the audience that a calibration problem had resulted in a whopping 20-degree error in pinpointing where the burst was on the sky. Turns out, the periodic signal had come from a regular pulsar in our galaxy. Known as PSR J0248+6021, it hails from the constellation Cassiopeia — a location much closer to the galactic plane. The observed dispersion measure can now be completely attributed to interstellar matter in the Milky Way.

You would think that professional astronomers know where their telescope is pointing. But CHIME is a weird instrument, with no moving parts. The sky position of a detected radio bursts is derived electronically from huge amounts of data – a process that needs to be precisely calibrated every day.

“We’ve only discovered the error last week,” says Michilli, “and we’re still figuring out the details. But it looks like we’ve been the victim of an extremely rare confluence of circumstances that messed up our calibration, including heavy rain and strong radio interference from a local source – possibly an airplane.”

After Michilli's announcement, the room burst into applause, and FRB scientist Joeri van Leeuwen (ASTRON, the Netherlands) complimented the team on their transparency. “If you make so many discoveries, as they do, there’s always a chance of one mistake,” he says. “Correcting your errors and preventing them from occurring again also helps the science go forward.”

Kaspi agrees. “Every challenge is a learning opportunity,” she says. “I believe we will come through stronger.”