Fast radio bursts, or FRBs, are one of the most exciting unsolved mysteries in astronomy today. FRBs are incredibly powerful bursts of short-wavelength radio waves from outer space. Each is the result of an event that lasted only a few milliseconds but produced up to 500 million times the energy produced by our sun over a comparable duration.
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 a West Virginia University astronomer who was examining old data from a radio telescope in Australia. The millisecond blip seemed to come from outside the universe.
Dozens of other FRBs were discovered later, but most were one-time events that were detected once and never repeated. This made FRBs impossible to predict, difficult to observe, and extremely difficult to trace back to a specific source. Without more information, researchers could only speculate on the astronomical events that might trigger such intense radio bursts.
A repetitive FRB
An FRB (named FRB 121102) was identified as repeating in 2016. Researchers pointed a large radio telescope at the location of a previously observed FRB and found that the signal repeated every few weeks or so, like the explains 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. Additionally, the fact that FRBs only last a few milliseconds means that the objects producing them cannot exceed 200 miles in diameter, which is the distance radio waves can travel during that time.
Therefore, the source of FRBs must be extremely dense and compact objects that are considerably smaller than an ordinary star. Possibilities included supernovae, 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 about 140 fast radio bursts. This number grew rapidly 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 the Earth spins. Digital image processing allows him to “look” in thousands of different directions at the same time.
Between 2018 and 2019, CHIME detected 535 FRBs. The analysis revealed that FRBs came from all over the universe and fell into two distinct categories. The majority of the 535 bursts were one-time events, while 61 were “repeaters” from 18 different sources. Repeaters typically lasted at least three times longer and emitted a much narrower band of radio frequencies.
These results suggest that FRBs are the result of at least two distinct astrophysical phenomena. Single FRBs are likely the result of cataclysmic events, such as the collision of two neutron stars or magnetic storms in young magnetars. Repeaters require more complex explanations, none of which have been verified.
Most repeaters are unpredictable, but two are known to cycle regularly. The first repeater ever identified (FRB 121102) is now known to cycle 90 days of FRB activity followed by 67 days of silence. This repeater is incredibly active; ScienceAlert reported that 122 bursts were detected in one hour. The second repeater with a regular cycle is named FRB 20180916B, which repeats every 16.35 days.
An FRB from inside the Milky Way
In April 2020, astronomers detected an FRB originating from our own galaxy. As explained in Astronomy, the magnetar SGR 1935+2154 began emitting X-rays from its location near the center of our galaxy about 30,000 light-years away. Eager to witness the spectacle, astronomers focused their telescopes and were able to pick up X-rays, gamma rays, and a fast radio burst named FRB 200428. The FRB lasted only 1.5 milliseconds, was the fastest FRB close ever detected and was 3,000 times brighter. than any previously observed magnetar radio signal.
While magnetars had been a prime candidate to explain FRBs, this was the first evidence that they could actually produce radio waves strong enough to accommodate signals. This Milky Way magnetar did not release as much energy as needed for FRBs millions or billions of light-years away, so it is possible that FRBs detected from outside our galaxy are from magnetars younger and more active.
FRB of galaxy spiral arms
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 between 400 million and 9 billion light-years away and are described as “[mostly] massive stars, relatively young and still forming.
This is consistent with the idea that FRBs come from young magnetar explosions. But just when it seemed the mystery of fast radio bursts was largely solved, the universe had another surprise in store.
A FRB from an old neighborhood
In 2021, National Geographic described how a repeating FRB (named FRB 20200120E) was assigned to a globular cluster, which is one of the oldest objects in the observable universe. Globular clusters are dense collections of very old stars and do not appear to contain the types of stars that can collapse into magnetars.
This discovery has forced astronomers to explain how a population of old, silent stars can generate such powerful explosions. Possible explanations include other types of stellar corpses, but there is currently no evidence to support this theory. This example FRB highlights the power of space radio waves that can come from a variety of sources. Identifying these sources will require powerful equipment, ingenious software and creative minds.
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