Astronomers from MIT report today that they have discovered a mysterious signal with a pattern akin to a heartbeat, emanating from a far-off galaxy that is billions of light-years from Earth. Exactly what the source may be of this regular pulse of radio waves remains a mystery, as it is the first time that such a signal has been recorded.
They have identified the signal as a fast radio burst (FRB), which is typically an intensely strong burst of radio waves of unknown astrophysical origin that lasts only a few milliseconds at most. This new signal, labelled FRB 20191221A, is unusual, because it persists for up to three seconds, which is about 1,000 times longer than the average FRB. Within this time, there are shorter bursts of radio waves that repeat every 0.2 seconds in a clear periodic pattern, similar to that of a beating heart.
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 that is located at the Dominion Radio Astrophysical Observatory in British Columbia, Canada. CHIME is designed to pick up radio waves emitted by hydrogen in the very earliest stages of the universe, but the telescope is also sensitive to fast radio bursts. Since it began observing the sky in 2018, CHIME has detected hundreds of FRBs emanating from different parts of the sky.
The vast majority of FRBs observed to date are ultrabright bursts of radio waves that last for a few milliseconds only. Recently, however, researchers 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 of the actual radio bursts was random rather than periodic.
On Dec. 21, 2019, CHIME picked up the signal of a potential FRB, which immediately drew the attention of Daniele Michilli, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research, who was scanning the incoming data.
“It was unusual,” he recalls. “Not only was it very long, lasting about three seconds, but there were periodic peaks that were remarkably precise, emitting every fraction of a second – boom, boom, boom – like a heartbeat. This is the first time the signal itself is periodic.”
Exactly what that source might be remains a mystery, though astronomers suspect the signal could emanate from either a radio pulsar or a magnetar, both of which are types of neutron stars – extremely dense, rapidly spinning collapsed cores of giant stars.
“There are not many things in the universe that emit strictly periodic signals,” says Michilli. “Examples that we know of in our own galaxy are radio pulsars and magnetars, which rotate and produce a beamed emission similar to a lighthouse. And we think this new signal could be a magnetar or pulsar on steroids.”
When they analyzed the pattern of FRB 20191221A’s radio bursts, Michilli and his colleagues found similarities with emissions from radio pulsars and magnetars in our own galaxy. The main difference, however, was that FRB 20191221A appears to be more than a million times brighter. Michilli says the luminous flashes may originate from a distant radio pulsar or magnetar that is normally less bright as it rotates and for some unknown reason ejected a train of brilliant bursts, in a rare three-second window that CHIME was luckily positioned to catch.
“CHIME has now detected many FRBs with different properties,” Michilli says. “We’ve seen some that live inside clouds that are very turbulent, while others look like they’re in clean environments. From the properties of this new signal, we can say that around this source, there’s a cloud of plasma that must be extremely turbulent.”
The team hopes to detect more periodic signals from this source, which could then be used as a type of astrophysical clock. For instance, the frequency of the bursts, and how they change as the source moves away from Earth, could be used to measure the rate at which the universe is expanding.
“This detection raises the question of what could cause this extreme signal that we’ve never seen before, and how can we use this signal to study the universe,” Michilli says. “Future telescopes promise to discover thousands of FRBs a month, and at that point we may find many more of these periodic signals.”
The discovery is reported today in the journal Nature, and is authored 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 then as a postdoc at MIT.