Astronomers discover mysterious repeated bursts of radio waves from outer space

Artist’s conception of a neutron star with an ultra-strong magnetic field, called a magnetar, emitting radio waves (red). Magnetars are a leading 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 ranging in length from a fraction of a millisecond to a few milliseconds, caused by some mysterious high-energy astrophysical process that has yet to be discovered. Astronomers estimate that the average FRB releases as much energy in one millisecond (one thousandth of a second) as the Sun in 3 days (which is over 250,000 seconds).

Duncan Lorimer and his student David Narkevic discovered the first FRB in 2007, and it is commonly known as the Lorimer Burst. Since then, many more 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 highly active and repetitive 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 usefulness 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, named FRB 190520, was found by the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) in China. An explosion of the object occurred on May 20, 2019, and was found in data from that telescope in November of that 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 optical image, when the FRB is bursting. Credit: Niu, et al.; Bill Saxton, NRAO/AUI/NSF; CFHT

Observations with the VLA in 2020 pinpointed the location of the object, and this allowed visible light observations with the Subaru telescope in Hawaii to show that it is on the outskirts of a dwarf galaxy nearly 3 billion light-years from Earth. The VLA observations also found that the object constantly emits weaker radio waves between bursts.

“These characteristics make this one very similar to the first FRB whose position was determined – also by the VLA – in 2016,” said Casey Law of Caltech. This development was a major step forward, providing the first information about the environment and distance of an FRB. However, its combination of repeated bursts and persistent radio emission between bursts, coming from a compact region, put the 2016 object, called FRB 121102, beyond all other known FRBs so far.

FRB 190520

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

“We now have two like this one, and that raises some important questions,” Law said. Law is part of an international team of astronomers who report their findings in the journal Nature.

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

“Are those who repeat different from those who do not? What about persistent radio emission – is this common?” said Kshitij Aggarwal, a graduate student at West Virginia University (WVU).

Astronomers suggest that there may be two different mechanisms producing FRBs, or that the objects that produce them may act differently at different stages of their evolution. Prime candidates for the sources of FRBs are superdense neutron stars left over after a massive star exploded as a supernova, or neutron stars with ultra-strong magnetic fields, called magnetars.

FRB 190520 Sky Map

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

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

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

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

“This means that there is a lot of material near the FRB that would confound any attempt to use it to measure gas between galaxies,” said Aggarwal. “If this is the case for others, we cannot count on using FRBs as cosmic measurements,” he added.

Astronomers have speculated that FRB 190520 may be a “newborn”, still surrounded by dense material ejected by the supernova explosion that left behind the star.

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