Astronomers at MIT and universities across Canada and the United States 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.
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 in the journal Nature, and is authored by members of the CHIME/FRB
Collaboration.
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.
In analyzing the pattern of FRB 20191221A’s radio bursts, the researchers 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.
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.
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.