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A source of Fast Radio Bursts (FRBs), which are transient radio pulses, is repeating itself in a 16-day cycle, scientists have found. The source, ‘FRB 180916.J0158+65’, was observed to be active for four days, and then nearly inactive for 12 days before the pattern repeated.
Although repeating FRBs have been observed before, the phenomenon was never found to follow any pattern. With the latest FRB, scientists claimed to have found 28 such cycles between September 2018 and October 2019.
FRBs are short, quick pulses of radio waves originating in far away regions of the Universe. They were first discovered in 2007. While the source of these pulses is still unknown, some researchers have hinted at an “extra-terrestrial source”. A majority of FRBs have been detected only once, because of which pinpointing their exact location within distant galaxies has remained a challenge. So far, only a few FRBs have been found to repeat cycles.
“Results suggest a mechanism for periodic modulation either of the burst emission itself, or through external amplification or absorption, and disfavour models invoking purely sporadic processes,” according to the pre-print of a research paper. Its authors indicated the source to be some unknown natural astrophysical process, said Abraham Loeb, chair of Harvard University's astronomy department.
His own guess was that the periodicity of the FRB originates from a binary star companion, or a long-term change in the orientation of the radio beam. “We observe periodicity in many astrophysical systems such as pairs of stars. Therefore, by itself, periodicity is not unusual enough to require an artificial origin. If we detected a Morse coded message, the data would have required a more creative interpretation,” said Loeb, the founding director of Harvard's Black Hole Initiative.
Loeb said:
FRBs are most likely bright analogs of pulsars, which are spinning neutron stars with strong magnetic fields. The duration and radio frequency of FRBs is similar to the flashes from pulsars, but they are intrinsically billions times brighter. As a result, we can see them from the edge of the Universe. Pulsars, on the other hand, are detectable only within the Milky Way galaxy.
He added that FRBs could also be associated with very young neutron stars — about a few decades old — with extremely strong magnetic fields called magnetars. However, according to Loeb, an artificial origin of the source should not be ruled out. He said that FRBs could be a “mixed population with a variety of source types”.
“There are some who suggest that the FRBs might be alien signals, but that really doesn’t make sense,” according to a September 2019 blog post by non-profit Search for Extraterrestrial Intelligence (SETI). The sources are spread all over intergalactic space, and arranging cooperative alien behaviour when even one-way communication takes many billions of years seems unlikely, SETI had said.
Loeb, however, presented a different argument: “The issue is not cooperation, since powerful signals are not necessarily used for communication. A civilisation could generate a powerful beam of light to propel cargos and the leak of that radiation could be detected.”
In one of his papers, Loeb explained that in order to produce FRBs across cosmological distances, one needs to use a huge amount of power comparable to the total power in sunlight intercepted by Earth. This would require a huge engineering project, far more ambitious than we currently have, he had said.
According to Loeb, a lot more data is still needed to study FRBs. One way to identify their nature is to discover a nearby repeating FRB which is bright, so that its environment could be studied in detail. “Another breakthrough in our understanding could originate from detecting FRBs at other wavelengths in addition to radio (optical, infrared or X-rays). This could provide an important clue about the nature of their central engine,” Loeb concluded.