Rapid-fire fast radio burst shows hot space between galaxies

Fast Radio Burst 20190520B -- a prolific repeating burst source -- was first observed in June 2019 by the Five-hundred-meter Aperture Spherical radio Telescope (FAST), in Ghizou province, southwest China. Astronomers generally consider this telescope as the spiritual successor to the now-defunct, Cornell University-built Arecibo Observatory in Puerto Rico.

After FAST found the burst, scientists then pinpointed the burst's location using the Very Large Array, Socorro, New Mexico.

What excites astronomers about the repeating fast radio bursts (FRBs) -- since they only burst once, generally speaking -- is that these quick-fire surges provide a pathway for scientists to comprehend the perplexing, mysterious and million-degree intergalactic medium.

"Examining the intergalactic medium is really hard," said co-author Shami Chatterjee, principal research scientist in astronomy at Cornell. "The intergalactic medium is difficult to probe, which is why fast radio bursts are exciting. The bursts let us study the properties of the intergalactic medium."

Joining Chatterjee on the Nature paper are James M. Cordes, professor of astronomy and Stella K. Ocker, a doctoral student in astronomy.

Co-author Di Li, is the chief scientist of both FAST and the radio division of the National Astronomical Observatories of Chinese Academy of Sciences. The observatory has discovered more than 100 pulsars and more than 5 FRBs.

Four bursts were detected during the initial 24-second scan in 2019, according to the paper. Between April and September 2020, during follow-up observations, FAST detected 75.

Due to the rapidly repeating bursts, astronomers believe that FRB 20190520B may be quite young. "It seems to reside in a complex plasma environment, like that expected in a young supernova remnant," Chatterjee said. "So one possibility is that the highly active source may be a newborn, and if so, it paints an intriguing evolutionary picture of FRB sources, where young burst sources are associated with persistent radio emission.

"The persistent emission fades away as the burst repetition rate slows down," Chatterjee said. "This is still very much a hypothesis and we are eager to test it with further examples of repeating FRBs."

Astronomers usually assume that FRBs pass through only a modest amount of gas (free electrons) in their host galaxies, which makes counting electrons in the intergalactic medium an easier task. FRB 20190520B shows the opposite: It has encountered far more gas in its host galaxy than scientists expected, calling into question previous assumptions.

Ultimately, astronomers want to know how the intergalactic medium is formed.

"We want to deconstruct how many free electrons are in the intergalactic medium, because it has been extremely difficult to study," Ocker said. "We don't know much about it."

"This particular new repeating FRB behaves in extreme, surprising ways," Ocker said.