Radio waves are part of the electromagnetic spectrum — which includes infrared light, visible light, ultraviolet light, X-rays and gamma rays — and, therefore, travel through space at the speed of light. The first FRB was discovered in 2007 by an astronomer at West Virginia University who was looking through old data from a radio telescope in Australia. The millisecond-long blip seemed to originate from outside the universe.
Dozens more FRBs
were later found, but most were one-off events that were detected once and
never repeated. This made FRBs impossible to predict, difficult to observe and
extremely challenging to trace to a specific source. Without more information,
researchers could only speculate about the astronomical events that might
trigger such intense radio bursts.
A Repeating FRB
One FRB (named FRB
121102) was identified as repeating in 2016. Researchers pointed a large radio
telescope to the location of a previously observed FRB and found that the
signal repeated every few weeks or so, as explained by the SETI Institute.
This repetition
helped scientists trace the source of the FRB to a dwarf galaxy 3 billion
light-years away. The massive distance traveled by this FRB indicated that it
was the result of an incredibly energetic event. Furthermore, the fact that
FRBs last only a few milliseconds means that the objects producing them can’t
be much bigger than 200 miles across, which is the distance that radio waves
can travel in that time.
Therefore, the
source of FRBs must be extremely dense, compact objects that are considerably
smaller than an ordinary star. Possibilities included supernovas, colliding
black holes and magnetars, which are collapsed stellar corpses that possess
the strongest magnetic fields in the universe.
More (Digital) Eyes on the Sky
Before 2017, astronomers
had detected a total of approximately 140 fast radio bursts. That number
quickly increased when the Canadian Hydrogen Intensity Mapping Experiment
(CHIME) telescope became operational. As explained in Nature,
CHIME is a giant telescope with no moving parts that scans the sky as Earth
rotates. Digital image processing allows it to “look” in thousands of different
directions at the same time.
Between 2018 and
2019, CHIME detected 535 FRBs. Analysis revealed that the FRBs originated from
across the universe and fell into two distinct categories. The majority of the
535 bursts were one-off events, while 61 were “repeaters” that came from 18
different sources. The repeaters typically lasted at least three times longer
and emitted a much narrower band of radio frequencies.
These findings
suggest that FRBs are the result of at least two distinct astrophysical
phenomena. The one-off FRBs are likely the result of cataclysmic events, such
as the collision of two neutron stars or magnetic storms in young magnetars.
The repeaters require more complex explanations, none of which have been
verified.
Most of the
repeaters are unpredictable, but two are known to have a regular cycle. The first
repeater ever identified (FRB 121102) is now known to follow a cycle of 90 days
of FRB activity followed by 67 days of silence. This repeater is incredibly
active; ScienceAlert reported that 122 bursts were detected in
a one-hour period. The second repeater with a regular cycle is named FRB
20180916B, which repeats every 16.35 days.
An FRB From Within the Milky Way
In April 2020,
astronomers detected an FRB that came from our own galaxy. As explained
in Astronomy, the magnetar SGR 1935+2154 started emitting
X-rays from its location near the center of our galaxy some 30,000 light-years
away. Eager to catch the show, astronomers focused their telescopes and were
able to catch X-rays, gamma rays and a fast radio burst that was named FRB 200428.
The FRB lasted just 1.5 milliseconds, was the closest FRB ever detected and was
3,000 times brighter than any previously observed magnetar radio signal.
While magnetars
had been a favored contender to explain FRBs, this was the first evidence that
they can actually produce radio waves powerful enough to account for the
signals. This Milky Way magnetar did not release as much energy as would be
required for the FRBs from millions or billions of light-years away, so it’s
possible that FRBs detected from outside our galaxy come from younger, more
active magnetars.
FRBs From the Spiral Arms of Galaxies
In May 2021,
astronomers used the excellent resolution of the Hubble Space Telescope to
trace the source of five FRBs to the spiral arms of five distant galaxies, as
reported by NASA. These galaxies are located 400 million to 9 billion
light-years away and are described as “[mostly] massive, relatively young and
still forming stars.”
This is consistent
with the idea that FRBs originate from young magnetar outbursts. But just when
it seemed that the mystery of fast radio bursts was mostly solved, the universe
had another surprise in store.
An FRB From an Old Neighborhood
In 2021, National Geographic described how a repeating FRB
(named FRB 20200120E) was traced to a globular cluster, which is one of the
most ancient objects in the observable universe. Globular clusters are dense
collections of very old stars and don’t seem to contain the kinds of stars that
can collapse into magnetars.
This discovery has forced astronomers to explain how a population of old, quiet stars can generate such powerful blasts. Possible explanations include other types of stellar corpses, but there’s currently no supporting evidence for that theory. This FRB example highlights how powerful radio waves from space might come from a variety of different sources. Identifying these sources will take powerful equipment, ingenious software and creative minds.