Radio waves

Astronomers discover mysterious repeated bursts of radio waves from outer space

Artist’s impression of a neutron star with an ultra-strong magnetic field, called a magnetar, emitting radio waves (red). Magnetars are a prime candidate for what generates fast radio bursts. Credit: Bill Saxton, NRAO/AUI/NSF

In radio astronomy, a fast radio burst (FRB) is a transient radio pulse lasting from a fraction of a millisecond to a few milliseconds, caused by a mysterious high-energy astrophysical process that has yet to be discovered. Astronomers estimate that the average FRB releases as much energy in a millisecond (one thousandth of a second) as the Sun emits in 3 days (more than 250,000 seconds).

Duncan Lorimer and his student David Narkevic discovered the first FRB in 2007, and it is commonly known as Lorimer Burst. Since then, many other FRBs have been detected. One of them, FRB 180916, is extremely mysterious because it pulses regularly every 16.35 days.

Now astronomers have found only the second example of a very active and repetitive fast radio burst with a compact source of weaker but persistent radio emission between bursts. The discovery raises new questions about the nature of these mysterious objects and also about their utility as tools for studying the nature of intergalactic space. Scientists used the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) and other telescopes to study the object, first discovered in 2019.

The object, called FRB 190520, was found by the Five Hundred Meter Aperture Spherical Radio Telescope (FAST) in China. A burst of the object occurred on May 20, 2019 and was found in data from this telescope in November of the same year. Follow-up observations with FAST have shown that, unlike many other FRBs, it emits frequent and repeated bursts of radio waves.

Fast radio burst VLA FRB 190520

VLA image of Fast Radio Burst FRB 190520 (red), combined with an optical image, when the FRB bursts. Credit: Niu, et al. ; Bill Saxton, NRAO/AUI/NSF; CFHT

Observations with the VLA in 2020 pinpointed the object’s location, allowing visible light observations with the Subaru Telescope in Hawaii to show it to be on the outskirts of a dwarf galaxy at nearly 3 billion light years from Earth. VLA observations also revealed that the object constantly emits weaker radio waves between bursts.

“These features make this one look a lot like the very first FRB whose position has been determined — also by the VLA — in 2016,” said Casey Law of Caltech. This development was a major breakthrough, providing the first environment and distance information of an FRB. However, its combination of repeated bursts and persistent radio emissions between bursts, originating from a compact region, set object 2016, called FRB 121102, apart from all other known FRBs, so far.

FRB 190520

The region of FRB 190520, seen in visible light, with the VLA image of the Fast Radio Burst animating between the bursting and bursting of the object. Credit: Niu, et al. ; Bill Saxton, NRAO/AUI/NSF; CFHT

“Now we have two like this, and it raises some important questions,” Law said. Law is part of an international team of astronomers reporting their findings in the journal Nature.

The differences between FRB 190520 and FRB 121102 and all the others reinforce a previously suggested possibility that there may be two different types of FRBs.

“Are those who rehearse different from those who don’t? What about persistent radio emission – is it common? said Kshitij Aggarwal, a graduate student at West Virginia University (WVU).

Astronomers suggest that there may be either two different mechanisms producing FRBs, or that the objects producing them may act differently at different stages of their evolution. Prime candidates for FRB sources are super-dense neutron stars that remain after a massive star explodes into a supernova, or neutron stars with ultra-strong magnetic fields, called magnetars.

FRB 190520 Chart of the sky

Location of FRB 190520 in the sky. Credit: Bill Saxton, NRAO/AUI/NSF

A feature of FRB 190520 calls into question the usefulness of FRBs as tools for studying matter between them and Earth. Astronomers often analyze the effects of intervening materials on radio waves emitted by distant objects to learn more about this tenuous material itself. One such effect occurs when radio waves pass through space containing free electrons. In this case, the high frequency waves travel faster than the low frequency waves.

This effect, called scattering, can be measured to determine the density of electrons in the space between the object and the Earth, or, if the electron density is known or assumed, to provide a rough estimate of the distance to the object. ‘object. The effect is often used to make distance estimates to pulsars.

This did not work for FRB 190520. An independent distance measurement based on the galaxy’s light Doppler shift caused by the expanding Universe placed the galaxy nearly 3 billion light-years away. of the earth. However, the signal from the burst shows an amount of scatter that would normally indicate a distance of around 8 to 9.5 billion light-years.

“That means there’s a lot of material near the FRB that would confuse any attempt to use it to measure gas between galaxies,” Aggarwal said. “If that’s the case with others, then we can’t rely on FRBs being used as cosmic criteria,” he added.

Astronomers have speculated that FRB 190520 could be a “newborn”, still surrounded by dense material ejected by the supernova explosion that left behind the neutron star. As this material dissipates, the dispersion of the burst signals would also decrease. Under the “newborn” scenario, they said, repeated bursts could also be a feature of younger FRBs and decrease with age.

“The FRB field is changing very rapidly right now, and new findings are coming out every month. However, big questions remain, and this object gives us some challenging clues to those questions,” said Sarah Burke-Spolaor of WVU.

Reference: “A repetitive rapid 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, CJ Law, S. Burke-Spolaor, JM Cordes, Y.-K. Zhang, SK Ocker, J.-M. Yao, P. Wan, Y. Feng, Y. Niino, C. Bochenek, M. Cruces, L. Connor, J.-A. Jiang, S. Dai, R. Luo , G.-D. Li, C.-C. Miao, J.-R. Niu, R. Anna-Thomas, J. Sydnor, D. Stern, W.-Y. Wang, M. Yuan, Y.-L. Yue, D.-J. Zhou, Z. Yan, W.-W. Zhu and B. Zhang, June 8, 2022, Nature.
DOI: 10.1038/s41586-022-04755-5

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under a cooperative agreement by Associated Universities, Inc.