Astronomers detect radio "heartbeat" billions of light years from Earth

Astronomers at MIT and universities in Canada and the United States have detected a strange and persistent radio signal from a distant galaxy that seems to flicker with surprising regularity.

The signal is classified as a rapid radio burst, or FRB — a very powerful burst of radio waves of unknown astrophysical origin, which usually lasts a few milliseconds at most. However, this new signal lasted up to three seconds, about 1,000 times longer than the FRB average. Within this window, the team detected bursts of radio waves that repeated every 0.2 seconds in a clear periodic pattern, similar to a heartbeat.

The researchers have labeled the signal FRB 20191221A, and it is currently the most durable FRB, with the clearest periodic pattern, detected to date.

The source of the signal lies in a distant galaxy, a few billion light years from Earth. Exactly what the source is remains a mystery, although astronomers suspect the signal could be from a radio pulsar or a magnetar, both types of neutron stars – the cores of giant stars that are extremely dense and rapidly rotating.

“There’s not much in the universe that emits such a strictly periodic signal,” said Daniele Michilli, a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research. “Examples that we know of in our own galaxy are radio pulsars and magnetars, which spin and produce beams similar to lighthouses. 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 can then be used as an astrophysical clock. For example, the frequency of the bursts, and how they change as the source moves away from Earth, can be used to measure the rate at which the universe is expanding.

The discovery was reported today in the journal Naturaland was written by members of the CHIME/FRB Collaboration, including MIT co-authors Calvin Leung, Juan Mena-Parra, Kaitlyn Shin, and Kiyoshi Masui at MIT, along with Michilli, who led the discovery first as a researcher at McGill University, and later as a postdoc at MIT. .

“Boom, boom, boom”

Since the first FRB was discovered in 2007, hundreds of similar radio flashes have been detected across the universe, most recently by the Canadian Hydrogen Intensity Mapping Experiment, or CHIME, an interferometric radio telescope consisting of four large parabolic reflectors located on the Dominion. Radio Astrophysics Observatory in British Columbia, Canada.

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

Most of the FRBs observed to date have been one-off — bursts of ultrabright radio waves lasting for a few milliseconds before flashing. Recently, researchers discovered the first periodic FRBs that appear to emit a regular pattern of radio waves. This signal consists of a four-day window of random bursts which are then repeated every 16 days. This 16-day cycle exhibits a periodic pattern of activity, although the radio burst signal is actually more random than periodic.

On December 21, 2019, CHIME caught a potential FRB signal, which immediately caught the attention of Michilli who was scanning the incoming data.

“It was unusual,” he recalls. “Not only was it incredibly long, lasting about three seconds, but there was a very precise periodic peak, emitting every split second — boom, boom, boom — like a heartbeat. This is the first time the signal itself is periodic.”

Brilliant burst

In analyzing the radio burst pattern of FRB 20191221A, Michilli and his colleagues found similarities to emissions from radio pulsars and magnetars in our own galaxy. A radio pulsar is a neutron star that emits a beam of radio waves, appearing to pulsate as the star rotates, while similar emissions are produced by a magnetar due to its extreme magnetic field.

The main difference between the new signal and radio emissions from our own galaxy’s pulsars and magnetars is that FRB 20191221A appears more than a million times brighter. Michilli said the flash of light may have come from a radio pulsar or a distant magnetar which is normally underexposed as it rotates and for some unknown reason emits a series of brilliant explosions, within a rare three-second window that CHIME was fortunately positioned to capture.

“CHIME has now detected many FRBs with different properties,” Michilli said. “We’ve seen some living in very turbulent clouds, while others look like they’re in a clean environment. From the nature of this new signal, we can tell that in the vicinity of this source, there is a plasma cloud that must be very turbulent.”

Astronomers hope to catch additional bursts from the periodic FRB 20191221A, which could help refine their understanding of their source, and of neutron stars in general.

“This detection raises questions about what could be causing these extreme signals we’ve never seen before, and how we can use these signals to study the universe,” Michilli said. “Future telescopes promise to find thousands of FRBs a month, and by then we may find many more of these periodic signals.”

This research was supported, in part, by the Canada Foundation for Innovation.

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