Fast radio bursts, or FRBs, are one of the most exciting unsolved mysteries in astronomy today. FRBs are incredibly powerful flares of short-wavelength radio waves from space. Each one is the result of an event that lasted just a few milliseconds but produced as much as 500 million times the energy our sun does over a comparable amount of time.
Fast Radio Burst (FRB) Basics
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.
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