A rapid radio burst (FRB) is a transient radio pulse with a duration ranging from a fraction of a millisecond to a few milliseconds that is created by an unknown high-energy astrophysical activity. Astronomers believe that the typical FRB emits as much energy in a millisecond (one thousandth of a second) as the Sun does in three days (which is over 250,000 seconds).
In 2007, Duncan Lorimer and his student David Narkevic found the first FRB, which became known as the Lorimer Burst. Many more FRBs have been discovered since then. FRB 180916 is particularly intriguing since it pulsates on a regular basis every 16.35 days.
Astronomers have discovered just the second instance of a highly active, repeated rapid radio burst accompanied with a compact source of weaker but persistent radio emission between bursts. The finding raises fresh concerns regarding the nature of these mystery objects, as well as their use as research instruments into the physics of intergalactic space. The scientists studied the object, which was discovered in 2019, using the National Science Foundation's Karl G. Jansky Very Large Array (VLA) and other telescopes.
The object, known as FRB 190520, was discovered by China's Five-hundred-meter Aperture Spherical Radio Telescope (FAST). The item erupted on May 20, 2019, and was discovered in data from that telescope in November of that year. FAST, unlike many other FRBs, releases numerous, repetitive bursts of radio waves, according to follow-up observations.
Studies with the VLA in 2020 identified the object's position, allowing visible-light observations with Hawaii's Subaru telescope to indicate that it sits on the fringes of a dwarf galaxy roughly 3 billion light-years from Earth. The VLA studies also revealed that the object produces weaker radio signals in between bursts.
"These features make this one appear a lot like the very first FRB whose location was identified — again by the VLA — back in 2016," stated Caltech's Casey Law. That achievement was significant since it provided the first information on a FRB's surroundings and distance. However, the 2016 item, known as FRB 121102, stood out from all previous known FRBs due to its combination of repetitive bursts and continuous radio emission between bursts emanating from a small location.
"Now we have two like this, which raises some critical problems," Law explained. Law is part of an international team of astronomers that published their results in Nature.
The discrepancies between FRB 190520 and FRB 121102, as well as the others, support an earlier suggestion that there may be two types of FRBs.
"Do those who repeat differ from those who do not?" Is it typical to have persistent radio emission?" Kshitij Aggarwal, a graduate student at West Virginia University, explained (WVU).
The scientists propose that FRBs are produced by two distinct mechanisms, or that the objects that produce them behave differently at various phases of their existence. The superdense neutron stars left over when a large star explodes as a supernova, or neutron stars with ultra-strong magnetic fields, known as magnetars, are leading possibilities for FRB sources.
One feature of FRB 190520 throws into doubt the use of FRBs as research instruments for investigating the material between them and Earth. Astronomers frequently study the effects of intervening material on radio waves radiated by faraway objects in order to understand more about that flimsy material. When radio waves flow across space containing free electrons, one such effect happens. Higher-frequency waves move faster than lower-frequency waves in this scenario.
This phenomenon, known as dispersion, can be studied to determine the density of electrons in the space between the object and Earth, or to offer an approximate estimate of the distance to the object if the electron density is known or assumed. The effect is frequently used to calculate distances to pulsars.
For FRB 190520, this did not work. An independent distance assessment based on the Doppler shift of the galaxy's light generated by the Universe's expansion places the galaxy at roughly 3 billion light-years from Earth. However, the burst's signal has a degree of dispersion that would normally suggest a distance of 8 to 9.5 billion light-years.
"This implies there is a lot of stuff close to the FRB that would complicate any effort to use it to quantify the gas between galaxies," Aggarwal explained. "If that's the case for others, we can't rely on FRBs as cosmic yardsticks," he continued.
The researchers hypothesised that FRB 190520 was a "newborn," still surrounded by dense material expelled by the supernova explosion that produced the neutron star. As that material finally disappears, so will the dispersion of the burst signals. According to the "newborn" scenario, the recurring bursts may also be a feature of younger FRBs and diminish with maturity.
"The FRB field is evolving at a breakneck pace right now, with new findings being announced on a monthly basis." "However, major questions remain, and this item is providing us with hard hints regarding those problems," said WVU's Sarah Burke-Spolaor.
"A repeating fast radio burst associated with a persistent radio source," by C.-H. Niu, K. Aggarwal, D. Li, X. Zhang, S. Chatterjee, C.-W. Tsai, W. Yu, C. J. Law, S. Burke-Spolaor, J. M. Cordes, Y.-K. Zhang, S. K. Ocker, J.-M. Yao, P. Wan,
DOI: 10.1038/s41586-022-04755-5
The National Radio Astronomy Observatory is a National Science Foundation facility managed by Associated Universities, Inc. under a cooperative agreement.
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