Astronomers discover 25 speedy radio bursts that repeat commonly

Like gravitational waves (GWs) and gamma-ray bursts (GRBs), fast radio bursts (FRBs) are one of the most powerful and mysterious astronomical phenomena today. These transient events consist of outbursts that give off more energy in one millisecond than the Sun does in three days. While most bursts only last milliseconds, there have been rare instances of FRBs repeating themselves. While astronomers are still unsure of what causes them and opinions vary, dedicated observatories and international collaborations have dramatically increased the number of events available for study.

A leading observatory is the Canadian Hydrogen Intensity Mapping Experiment (CHIME), a next-generation radio telescope at the Dominion Radio Astrophysical Observatory (DRAO) in British Columbia, Canada. Thanks to its large field of view and wide frequency coverage, this telescope is an indispensable tool for detecting FRBs (more than 1000 sources to date!). Using a novel algorithm, the CHIME/FRB collaboration found evidence of 25 new repeating FRBs in CHIME data collected between 2019 and 2021.

The CHIME/FRB collaboration includes astronomers and astrophysicists from Canada, the US, Australia, Tawain and India. Its partner institutions include DRAO, Dunlap Institute for Astronomy and Astrophysics (DI), Perimeter Institute for Theoretical Physics, Canadian Institute for Theoretical Astrophysics (CITA), Anton Pannekoek Institute for Astronomy, National Radio Astronomy Observatory (NRAO) , the Institute of Astronomy and Astrophysics, the National Center for Radio Astrophysics (NCRA) and the Tata Institute of Fundamental Research (TIFR), as well as several universities and institutes.

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Despite their mysterious nature, FRBs are ubiquitous and the best estimates show that events arrive on Earth about a thousand times a day across the sky. None of the theories or models proposed so far can fully explain all the properties of the eruptions or the sources. While some are believed to be caused by neutron stars and black holes (due to the high energy density of their surroundings), others continue to elude classification. Because of this, other theories exist ranging from pulsars and magnetars to GRBs and extraterrestrial communications.

CHIME was originally developed to measure the expansion history of the universe by detecting neutral hydrogen. About 370,000 years after the Big Bang, the universe was permeated by this gas, and the only photons were either relic radiation from the Big Bang – the Cosmic Microwave Background (CMB) – or that released by neutral hydrogen atoms. For this reason, astronomers and cosmologists refer to this period as the “Dark Ages,” which ended approximately 1 billion years after the Big Bang, when the first stars and galaxies began to reionize neutral hydrogen (the Age of Reionization).

In particular, CHIME was designed to detect the wavelength of light that neutral hydrogen absorbs and emits, known as the 21-centimeter hydrogen line. This allowed astronomers to measure how fast the universe was expanding during the “dark ages” and make comparisons with later observable cosmological epochs. However, CHIME has now proven to be ideally suited for investigating FRBs due to its wide field of view and frequency range covered (400 to 800 MHz). This is the goal of the CHIME/FRB collaboration, which is to detect, characterize and trace FRBs to their sources.

As Dunlap Postdoctoral Fellow and lead author Ziggy Pleunis told Universe Today, each FRB is described by its position in the sky and a quantity known as the dispersion measure (DM). This refers to the time delay from high to low frequencies caused by the burst’s interactions with material as it travels through space. In a paper published in August 2021, the CHIME/FRB collaboration presented the first large-sample catalog of FRBs, containing 536 events detected by CHIME between 2018 and 2019, including 62 bursts from 18 previously reported repetitive sources.

Artist’s rendering of a rapid radio burst and the observatories dedicated to detecting it. Photo credit: Danielle Futselaar

For this latest study, Pleunis and his colleagues relied on a new clustering algorithm that looks for multiple events that are in the sky with similar DMs. “We can measure the sky position and the scatter of the fast radio burst to a certain accuracy, which depends on the design of the telescope used,” Pleunis said. “The clustering algorithm takes into account all fast radio bursts detected by the CHIME telescope and looks for clusters of FRBs that have consistent sky positions and dispersion measures within the measurement uncertainties. We then run various checks to ensure that the bursts in a cluster really do come from the same source.”

Of the over 1000 FRBs discovered to date, only 29 have been identified as repetitive. Additionally, it has been found that virtually all repeating FRBs repeat in an irregular fashion. The only exception is FRB 180915, discovered by researchers at CHIME in 2018 (and reported in 2020), which pulses every 16.35 days. Using this new algorithm, the CHIME/FRB collaboration discovered 25 new repetitive sources, nearly doubling the number available for study. In addition, the team found some very interesting features that could provide insight into their causes and properties. As Pleunis added:

“If we carefully count all of our fast bursts and the sources that are repeated, we find that only about 2.6% of all fast bursts we detect are repeated. For many of the new sources we only detected a few bursts, making the sources fairly inactive. Almost as dormant as the springs we’ve only seen once.

“We can therefore not rule out that the sources from which we have only seen one burst so far will also show repeat bursts at some point. It is possible for all fast radio burst sources to repeat at some point, but many sources are not very active. Any explanation for fast radio bursts should explain why some sources are hyperactive while others are mostly quiet.”

An illustration of CHIME detecting Fast Radio Bursts (FRBs) in the night sky. Credit: James Josephides/Mike Dalley

These results could help inform future surveys that will benefit from next-generation radio telescopes that will become operational in the years to come. These include the Square Kilometer Array Observatory (SKAO), which is expected to collect its first light by 2027. Located in Australia, this 128 dish telescope will be merged with the MeerKAT array in South Africa to create the largest radio telescope in the world. In the meantime, the amazing rate at which new FRBs are being discovered (including repeating events) could mean that radio astronomers could be on the verge of a breakthrough!

Further reading: arXiv

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