Cosmic cat and mouse: Astronomers Capture and Tag a Fleeting Radio Burst

An Australian-led team of astronomers using the Gemini South telescope in Chile have successfully confirmed the distance to a galaxy hosting an intense radio burst that flashed only once and lasted but a thousandth of a second. The team made the initial discovery of the fast radio burst (FRB) using the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope.

The critical Gemini observations were key to verifying that the burst left its host galaxy some 4 billion years ago.

Since the first FRB discovery in 2007, these mysterious objects have played a game of cosmic cat-and-mouse with astronomers -- with astronomers as the sharp-eyed cats! Fleeting radio outbursts, lasting about a millisecond (one-thousandth of one second), are difficult to detect, and even more difficult to locate precisely. In this case, the FRB, known as FRB 180924, was a single burst, unlike others that can flash multiple times over an extended period.

"It is especially challenging to pinpoint FRBs that only flash once and are gone," said Keith Bannister of Australia's Commonwealth Science and Industrial Research Organisation (CSIRO), who led the Australian team in the search effort. However, Bannister and his team did just that, which is a first. 

The result is published in the June 27th issue of the journal Science.

The momentary pulse was first spotted in September 2018 during a dedicated search for FRBs using ASKAP -- a 36-antenna array of radio telescopes working together as a single instrument in Western Australia -- which also pinpointed the signal's location in the sky. 

The researchers used the minuscule differences in the amount of time it takes for the light to reach different antennas in the array to zoom in on the host galaxy's location. "From these tiny time differences -- just a fraction of a billionth of a second -- we identified the burst's home galaxy," said team member Adam Deller, of the Swinburne University of Technology. 

Once pinpointed, the team enlisted the Gemini South telescope, along with the W.M. Keck Observatory and European Southern Observatory's Very Large Telescope (VLT) to determine the FRB's distance and other characteristics by carefully observing the galaxy that hosted the outburst. "The Gemini South data absolutely confirmed that the light left the galaxy about 4 billion years ago," said Nicolas Tejos of Pontificia Universidad Católica de Valparaíso, who led the Gemini observations. 

"ASKAP gave us the two-dimensional position in the sky, but the Gemini, Keck, and VLT observations locked down the distance, which completes the three-dimensional picture," said Tejos.

"When we managed to get a position for FRB 180924 that was good to 0.1 arcseconds, we knew that it would tell us not just which object was the host galaxy, but also where within the host galaxy it occurred," said Deller. "We found that the FRB was located away from the galaxy's core, out in the 'galactic suburbs.'"

"The Gemini telescopes were designed with observations like this in mind," said Ralph Gaume, Deputy Division Director of the US National Science Foundation (NSF) Division of Astronomical Sciences, which provides funding for the US portion of the Gemini Observatory international partnership. Knowing where an FRB occurs in a galaxy of this type is important because it enables astronomers to get some hint of what the FRB progenitor might have been. "And for that," Gaume continues, "we need images and spectroscopy with superior image quality and depth, which is why Gemini and the optical and infrared observatory observations in this study were so important."

Localizing FRBs is critical to understanding what causes the flashes, which is still uncertain: to explain the high energies and short timescales, most theories invoke the presence of a massive yet very compact object such as a black hole or a highly magnetic neutron star. Finding where the bursts occur would tell us whether it is the formation, evolution, or collision and destruction of these objects that is generating the radio bursts."

"Much like gamma-ray bursts two decades ago, or the more recent detection of gravitational wave events, we stand on the cusp of an exciting new era where we are about to learn where fast radio bursts take place," said team member Stuart Ryder of Macquarie University, Australia. Ryder also noted that by knowing where within a galaxy FRBs occur, astronomers hope to learn more about what causes them, or at least rule out some of the many models. "Ultimately though," Ryder continued, "our goal is to use FRBs as cosmological probes, in much the same way that we use gamma-ray bursts, quasars, and supernovae." According to Ryder, such a map could pinpoint the location of the 'missing baryons,' (baryons are the subatomic building blocks of matter) which standard models predict must be out there, but which don't show up using other probes.

By pinpointing the bursts and how far their light has traveled, astronomers can also obtain "core samples" of the intervening material between us and the flashes. With a large sample of FRB host galaxies, astronomers could conduct "cosmic tomography,"' to build the first 3D map of where baryons are located between galaxies. On that note, Tejos added, "once we have a large sample of FRBs with known distances, we will also have a revolutionary new method for measuring the amount of matter in the cosmic web!"

To date, only one other fast radio burst (FRB 121102) has been localized, and it had a repeating signal that flashed more than 150 times, While both single and multiple flash FRBs are relatively rare, single FRBs are more common than repeating ones. The discovery of FRB 180924, then, could lead the way for future methods of localization.

"Fast turnaround follow-up contributions from Gemini Observatory will be especially significant in the future of time-domain astronomy," Tejos said, "as it promises not only to help astronomers perfect the study of transient phenomena but perhaps alter our perceptions of the Universe."