A Rare Repeating Fast Radio Burst Gives Astronomers a Chance to Study These Mysterious Objects

Fast Radio Bursts (FRBs) are among the most mysterious astronomical phenomena facing astronomers today. While hundreds of bursts have been detected since the first-ever recorded detection of an FRB in 2007 – the Lorimer Burst – astronomers are still unsure what causes them. Even more mysterious, some have occasionally been found to be repeating in nature, which has fueled speculation that they may not be natural in origin (i.e., possible alien transmissions?). Astronomers are naturally very excited whenever a repeating FRB is found, as it gives them the chance to examine them closer.

In a recent survey, an international team of scientists used three major telescopes worldwide to study a repeating FRB (known as FRB 190520) that was first observed in 2019. According to their observations, this particular FRB is not just a repeating source from a compact object but a persistent one that emits low-level bursts of radio waves between larger ones. These findings raise new questions about the nature of these mysterious objects and how they can be used as tools to probe the space between stars and galaxies.

The study that describes their findings, titled “A repeating fast radio burst associated with a persistent radio source,” was recently published in the journal Nature. The research was led by Catherine Hui Niu and Di Li of the National Astronomical Observatories (NAO) at the Chinese Academy of Sciences (CAS) in Beijing and Kaspar Aggarwal of the Center for Gravitational Waves and Cosmology at West Virginia University. They were joined by researchers from Caltech, the Canadian Institute for Advanced Research, Cornell Center for Astrophysics and Planetary Science, and multiple universities and observatories.

A Mystery

FRBs are short-lived, intense blasts of radio energy that typically last for a few milliseconds and are never heard from again. The first FRB was detected in 2007 by Duncan Lorimer, an astronomer at West Virginia University (hence the name “Lorimer Burst “) and several hundred have been detected ever since. Whereas most FRBs detected have been one-off events, a few have been repeating in nature and have even been traced back to their sources.

The study of FRBs has advanced considerably thanks to the creation of next-generation radio observatories like China’s Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the Canadian Hydrogen Intensity Mapping Experiment (CHIME) – which commenced operations in 2016 and 2017 (respectively). Thanks to these facilities, astronomers have narrowed the list of possible causes and can now detect hundreds of bursts per year and track FRBs in real-time.

Because of these efforts, astronomers have found a strong link between repeating FRBs and magnetars, a highly-magnetized type of neutron star. Combined with global coordination efforts, astronomers have gathered a huge amount of data on these transient objects to the point that their transient nature is no longer impeding research into them.

Two of a Kind

FRB 190520 was first noticed by scientists at China’s FAST array on May 20th, 2019, after sifting through data obtained in November of that year. Follow-up observations conducted with FAST immediately afterward showed that this source was repeating in nature. In 2020, observations with the Karl G. Jansky Very Large Array (VLA) pinpoint the object’s location, while visible-light observations with the Subaru Telescope in Hawaii showed that this placed it in the outskirts of a dwarf galaxy nearly 3 billion light-years from Earth.

The data obtained with the VLA also determined that the object constantly emits weaker radio waves between bursts. These observations provided the first information about the environment and distance of an FRB, which constituted a major breakthrough in the study of these objects. However, the combination of repeating bursts and persistent radio emissions from a compact region meant that this discovery was only the second of its kind. As Caltech astronomer (and study co-author) Casey Law explained, the only other object of this kind was FRB 121102, which was spotted in 2016.

“These characteristics make this one look a lot like the very first FRB whose position was determined — also by the VLA — back in 2016,” she said. “Now we have two like this, and that brings up some important questions.” The differences between these two sources and all others detected to date strengthen the possibility that there may be two distinct types of FRBs, something astronomers have long suspected.

Possible Explanations

The team proposed that these results could indicate one of two things. First, there’s the possibility that there may be different mechanisms producing the two types of FRBs observed (single events and repeating) or that the objects producing them may be at different stages in their evolution. As noted, the leading candidates for FRBs are thought to be neutron stars with ultra-strong magnetic fields – called magnetars. But after examining FRB 190520, they came to the tentative conclusion that it may be a “newborn” neutron star.

Essentially, this means that the neutron star is still surrounded by dense clouds of dust and gas, which are the remains of the star’s outer layers that were ejected when it went supernova. The presence of this material, they claim, would explain the effect the source’s radio waves had on the surrounding environment. Like pulsars, FRBs affect the dust and gas that lies between star systems and galaxies, which astronomers can study to learn more about the material. For instance, when radio waves pass through space containing free electrons, higher-frequency waves travel faster than low-frequency waves.

The region of FRB 190520 with the VLA image of the Fast Radio Burst alternating between the object bursting and not bursting. Credit: Niu, et al.; Bill Saxton, NRAO/AUI/NSF; CFHT

This effect is known as “dispersion” and can be measured to determine the density of electrons in interstellar or intergalactic space. In instances where the density of electrons in the intervening space is known, FRBs can also be used to determine the distance between a source and Earth. When the team attempted to make distance measurements based on the dispersion effect FRB 190520 had on the surrounding gas and dust, the results indicated a distance of roughly 8 to 9.5 billion light-years. However, independent measurements based on the source’s Doppler shift produced an estimate of almost 3 billion light-years.

This explanation has multiple implications for the study of FRBs. For one, it demonstrates that repeating bursts could be characteristic of younger neutron stars and that these FRBs dwindle with age as the gas and dust clouds surrounding them dissipate. Second, it raises questions about how useful FRBs could be in determining the distances between celestial objects and the density of the space between them. Said Aggarwal:

“This means that there is a lot of material near the FRB that would confuse any attempt to use it to measure the gas between galaxies. If that’s the case with others, then we can’t count on using FRBs as cosmic yardsticks.”

While it is clear that many questions remain about the sources and mechanisms of Fast Radio Bursts, the progress being made in this field is astounding. Only fifteen years ago, astronomers had observed one for the first time and didn’t realize they came in two distinct forms. Today, FRBs are being detected and tracked by the hundreds, and astronomers are getting closer to determining the key characteristics of both types. In this sense, the field of FRB research is right up there with research into Gravitational Waves (GW), infrared astronomy, exoplanet studies, astrobiology, and others that are progressing by leaps and bounds!

Further Reading: NRAO