An international team of researchers has discovered a strange, repeated rapid radio burst terminating from a distant dwarf galaxy around three billion light-years away. According to the new study the bursts stand out very clearly when compared to previous radio burst detections in recent years.
The bursts, called fast radio bursts or FRBs for short, are millisecond-long radio wave bursts in space. Individual radio bursts are only broadcast once and are not repeated. However, repeating rapid radio bursts are known to put out several short, intense radio waves.
Some of these radio bursts have been traced back to their source galaxies, but the exact cause of the pulses has yet to be determined. This would be critical in explaining how they are generated.
Astronomers were able to detect the bursts, known as FRB 190520B, back on May 20, 2019. To find out more, the researchers then used the FAST (Five-hundred-meter Aperture Spherical radio Telescope) in China in November of the same year. When they did this, the scientists detected something strange about the phenomenon — the object was emitting regular, repetitive bursts of radio waves.
To try to disentangle the problem further, researchers then used the Karl G. Jansky Very Large Array, or VLA, telescopes of the National Science Foundation in 2020 to determine the burst's origin before zeroing down on it using the Subaru Telescope in Hawai'i in 2021. Subaru's visible-light scans revealed the explosion originated on the fringes of a faraway dwarf galaxy.
The results were detailed in a study published in the journal Nature on Wednesday 9 June, 2022.
FRB 190520B is fairly unique
Between the recurring bursts, the astronomical object continuously produced weaker radio waves, according to the VLA data. Only one other known repeating fast radio burst has been detected to date, FRB 121102, which was discovered in 2016.
The discovery of FRB 121102 and subsequent tracking of it back to its starting point in a tiny dwarf galaxy almost three billion light-years distant was a watershed moment in astronomy. It was the first time astronomers had been able to determine the distance and environment of these enigmatic objects.
"Now we actually need to explain this double mystery and why FRBs and persistent radio sources are found together sometimes," explained study co-author Casey Law, a staff scientist in radio astronomy at the California Institute of Technology. "Is it common when FRBs are young? Or perhaps the object that makes the bursts is a massive black hole that is messily eating up a neighboring star? Theorists have a lot more detail to work with now and the scope for explanation is shrinking," he added.
Currently, less than five percent of the hundreds of identified fast radio bursts have been known to repeat and only a few of them are regularly active.
However, FRB 190520B is the only constantly active example yet found, meaning it has never "turned off" since being discovered, said study author Di Li, chief scientist for the radio division of the National Astronomical Observatories of China and the FAST Operation Center. Meanwhile, FRB 121102, "the first known famous repeater, can turn off for months," Li said.
The discovery creates more questions than answers about FRBs
From the results of this study, astronomers are now wondering whether or not there are two types of rapid radio bursts, rather than a one-size-fits-all kind.
"Are those that repeat different from those that don't? What about the persistent radio emission — is that common?" said study coauthor Kshitij Aggarwal, who was involved in the study as a doctoral student at West Virginia University, in a statement.
It's conceivable, for example, that the radio bursts are caused by distinct processes, or that whatever causes them behaves differently at different phases of development.
Fast radio bursts had previously been thought to be created by dense remains of supernovae, released from neutron stars, or created by neutron stars with extraordinarily powerful magnetic fields, known as "magnetars."
"A coherent picture of the origin and evolution of FRBs is likely to emerge in just a few years," Li said.
With the possibility of finding an entirely new class of radio wave classes, Li is understandably very excited.
"For decades, astronomers thought there were basically two kinds of radio source that we could see in other galaxies: accreting supermassive black holes and star formation activity," Law explained. "Now we're saying that it can't be an either/or categorization anymore! There is a new kid in town and we should consider that when studying populations of radio sources in the universe," he added.
"The dispersive sweep of fast radio bursts (FRBs) has been used to probe the ionized baryon content of the intergalactic medium, which is assumed to dominate the total extragalactic dispersion. Although the host-galaxy contributions to the dispersion measure appear to be small for most FRBs, in at least one case there is evidence for an extreme magneto-ionic local environment and a compact persistent radio source. Here we report the detection and localization of the repeating FRB 20190520B, which is co-located with a compact, persistent radio source and associated with a dwarf host galaxy of high specific-star-formation rate at a redshift of 0.241 ± 0.001. The estimated host-galaxy dispersion measure of approximately 903+72−111903−111+72 parsecs per cubic centimeter, which is nearly an order of magnitude higher than the average of FRB host galaxies, far exceeds the dispersion-measure contribution of the intergalactic medium. Caution is thus warranted in inferring redshifts for FRBs without accurate host-galaxy identifications."