Strange and persistent radio "heartbeat" detected billions of light-years from Earth

Astronomers at MIT and other centers have detected a strange and persistent radio signal from a distant galaxy that seems to flicker with "surprising" regularity, according to a press release. The signal is classified as a Fast Radio Burst, or FRB, for short, an intensely strong burst of radio waves of unknown astrophysical origin, typically lasting a few milliseconds at most. However, this new signal persists for up to three seconds, about a thousand times longer than the FRB average.

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Within this window, the team has detected bursts of radio waves that repeat every 0.2 seconds in a clear periodic pattern, similar to that of a beating heart. The researchers have labeled the signal FRB 20191221A, and it is currently the longest-lasting FRB, with the clearest periodic pattern, detected to date. The discovery is published today in the journal Nature.

The source of the signal, MIT sources indicate in their statement, is located in a distant galaxy, several billion light years from Earth. The exact source remains a mystery, though astronomers suspect the signal could come from a radio pulsar or a magnetar, both types of neutron stars, extremely dense and rapidly rotating collapsed cores of giant stars.

"There aren't many things in the universe that emit strictly periodic signals," says Daniele Michilli, a postdoctoral researcher at MIT's Kavli Institute for Astrophysics and Space Research. “The examples we know of in our own galaxy are radio pulsars and magnetars, which rotate and produce an emission similar to that of a lighthouse. And we think this new signal could be a magnetar or pulsar on steroids."

The team hopes to detect more periodic signals from this source, which could be used as an astrophysical clock. For example, the frequency of the outbursts, and how they change as the source moves away from Earth, could be used to measure the rate of expansion of the universe.

"Boom, boom, boom"

Since the first FRB was discovered in 2007, hundreds of similar radio flashes have been detected across the universe, most recently detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) thanks to an interferometric radio telescope that combines the signals received by four large parabolic reflectors located at the Radio Dominion Astrophysical Observatory in the Canadian region of British Columbia.

The CHIME project continuously observes the sky as the Earth rotates, and is designed to capture radio waves emitted by hydrogen in the earliest stages of the universe. The telescope is also sensitive to fast radio bursts, and since it began observing the sky in 2018, it has detected hundreds of FRBs emanating from different parts of the sky.

The vast majority of FRBs observed to date are punctual: ultra-bright bursts of radio waves that last a few milliseconds before fading out. Researchers recently discovered the first periodic FRB that appeared to emit a regular pattern of radio waves. This signal consisted of a four-day window of random bursts that then repeated every 16 days. This 16-day cycle indicated a periodic pattern of activity, although the signal from the actual radio bursts was random rather than periodic.

On December 21, 2019, a signal from a potential FRB was picked up, which immediately caught the attention of Michilli, who was scanning the incoming data. "It was unusual," he remembers. “Not only was it very long, lasting about three seconds, but there were periodic spikes that were remarkably precise, making each fraction of a second – thump, thump, thump – sound like a heartbeat.” It is the first time that the signal itself is periodic."

bright bursts

By analyzing the pattern of radio bursts from FRB 20191221A, Michilli and colleagues found similarities to emissions from radio pulsars and magnetars in our own galaxy. Radio pulsars are neutron stars that emit beams of radio waves that appear to pulse as the star rotates, while magnetars produce similar emission due to their extreme magnetic fields.

The main difference between the new signal and radio emissions from our own galactic pulsars and magnetars is that FRB 20191221A appears to be more than a million times brighter. Michilli says the flashes of light may originate from a distant radio pulsar or magnetar that is normally dimmer as it rotates and that, for some unknown reason, ejected in just three seconds a cluster of bright bursts that the observatory was fortunate to catch.

"The CHIME project has now detected many FRBs with different properties," says Michilli. “We have seen some that live inside clouds that are very turbulent, while others appear to be in clean environments.” From the properties of this new signal, we can say that around this source there is a plasma cloud that must be extremely turbulent."

Astronomers hope to catch additional bursts from the periodic FRB 20191221A, which may help refine their understanding of its source, and of neutron stars in general. "This detection raises the question of what could cause this extreme signal that we have never seen before, and how can we use this signal to study the universe," says Michilli. "Future telescopes promise to discover thousands of FRBs a month, and by that time we could find many more of these periodic signals."

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