Astronomers at MIT and elsewhere have detected a strange and persistent
radio signal from a far-off galaxy that appears to be flashing with
surprising regularity.
The signal is classified as a fast radio burst, or FRB—an intensely strong
burst of radio waves of unknown astrophysical origin, that typically lasts
for a few milliseconds at most. However, this new signal persists for up to
three seconds, about 1,000 times longer than the average FRB. Within this
window, the team detected bursts of radio waves that repeat every 0.2
seconds in a clear periodic pattern, similar to 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 source of the signal lies in a distant galaxy, several billion
light-years from Earth. 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 Daniele Michilli, a postdoc in MIT's Kavli Institute for
Astrophysics and Space Research. "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."
The team hopes to detect more periodic signals from this source, which could
then be used as an 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.
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.
"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 that is
located at the Dominion Radio Astrophysical 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 very earliest stages of the
universe. The telescope also happens to be sensitive to fast radio bursts,
and 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 one-offs—ultrabright bursts
of radio waves that last for a few milliseconds before blinking off.
Recently, 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, though the signal of the actual
radio bursts was random rather than periodic.
On Dec. 21, 2019, CHIME picked up a signal of a potential FRB, which
immediately drew the attention of Michilli, 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."
Brilliant bursts
In analyzing 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. Radio pulsars are neutron stars that emit beams
of radio waves, appearing to pulse as the star rotates, while a similar
emission is produced by magnetars 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 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 astronomers hope to catch additional bursts from the periodic FRB
20191221A, which can help to 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'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."
Reference:
Daniele Michilli, Sub-second periodicity in a fast radio burst, Nature
(2022).
DOI: 10.1038/s41586-022-04841-8.
Tags:
Space & Astrophysics