A team of astronomers, including researchers from MIT, has picked up a curious, repetitive rhythm of fast radio bursts emanating from an unknown source outside our galaxy, 500 million light-years away.
Fast radio bursts, or FRBs, are short, intense flashes of radio waves thought to be the product of small, distant, extremely dense objects, although the exact nature of these objects is a long-standing mystery in astrophysics. . FRBs typically last a few milliseconds, during which time they can eclipse entire galaxies.
Since the first FRB was observed in 2007, astronomers have cataloged more than 100 fast radio bursts from distant sources scattered across the universe, outside of our own galaxy. For the most part, these detections were spot-on, flashing briefly before disappearing completely. In a few cases, astronomers have observed fast radio bursts multiple times from the same source, but with no discernible pattern.
This new FRB source, which the team has cataloged as FRB 180916.J0158+65, is the first to produce a periodic or cyclic pattern of fast radio bursts. The model begins with a noisy four-day window, during which the source emits random bursts of radio waves, followed by a 12-day period of radio silence.
Astronomers observed that this 16-day pattern of fast radio bursts repeated itself regularly over 500 days of observations.
“This FRB we’re reporting now is like clockwork,” says Kiyoshi Masui, assistant professor of physics at MIT’s Kavli Institute for Astrophysics and Space Research. “This is the most definitive model we’ve seen of any of these sources. And it’s a big clue that we can use to start looking at the physics of what’s causing these bright flashes, which no one understands. really.
Masui is a member of the CHIME/FRB Collaboration, a group of more than 50 scientists led by the University of British Columbia, McGill University, University of Toronto and the National Research Council of Canada, which operates and analyzes data from the Canadian Hydrogen Intensity Mapping Experiment, or CHIME, a radio telescope in British Columbia that was the first to pick up signals from the new periodic FRB source.
The CHIME/FRB collaboration published details of the new observation today in the journal Nature.
A radio view
In 2017, CHIME was erected at the Dominion Radio Astrophysical Observatory in British Columbia, where it quickly began detecting fast radio bursts from galaxies across the universe, billions of light-years from Earth. .
CHIME consists of four large antennas, each the size and shape of a snowboard half-pipe, and is designed with no moving parts. Rather than rotating to focus on different parts of the sky, CHIME stares at the entire sky, using digital signal processing to pinpoint the region of space where incoming radio waves are coming from.
From September 2018 to February 2020, CHIME selected 38 fast radio bursts from a single source, FRB 180916.J0158+65, which astronomers traced to a star-churning region on the outskirts of a spiral galaxy. massive, 500 million light-years from Earth. The source is the most active FRB source that CHIME has yet detected, and until recently it was the closest FRB source to Earth.
As the researchers plotted each of the 38 bursts over time, a pattern began to emerge: one or two bursts would occur over four days, followed by a 12-day period without any bursts, after which the pattern would repeat itself. This 16-day cycle happened over and over again during the 500 days they observed the source.
“These periodic bursts are something we’ve never seen before, and it’s a new phenomenon in astrophysics,” Masui says.
The exact phenomenon behind this new extragalactic rhythm is a big unknown, though the team is exploring some ideas in their new paper. One possibility is that the periodic bursts could originate from a single compact object, such as a neutron star, that both spins and oscillates – an astrophysical phenomenon known as precession. Assuming that the radio waves emanate from a fixed location on the object, if the object rotates along an axis and that axis is only pointed in the direction of the Earth every four out of 16 days, then we would observe radio waves as periodic. bursts.
Another possibility involves a binary system, such as a neutron star orbiting another neutron star or a black hole. If the first neutron star is emitting radio waves and is in an eccentric orbit that briefly brings it closer to the second object, the tides between the two objects could be strong enough to cause the first neutron star to warp and briefly erupt before she does not walk away. . This pattern would repeat as the neutron star returned along its orbit.
The researchers considered a third scenario, involving a source of radio emission that surrounds a central star. If the star is emitting a wind or a cloud of gas, then each time the source passes through the cloud, the gas from the cloud could periodically amplify the source’s radio emissions.
“Maybe the source is still emitting these gusts, but we only see them when it passes through these clouds, because the clouds act like a lens,” Masui says.
Perhaps the most exciting possibility is the idea that this new FRB, and even those that are not periodic or even repeating, may have originated from magnetars – a type of neutron star that is thought to have a field extremely powerful magnet. The details of magnetars are still a bit of a mystery, but astronomers have observed that they sometimes release massive amounts of radiation across the electromagnetic spectrum, including energy in the radio band.
“People have been working on how to get these magnetars to emit fast radio bursts, and this periodicity that we observed has since been incorporated into these models to understand how it all fits together,” says Masui.
Very recently, the same group made a new observation that supports the idea that magnetars might in fact be a viable source of fast radio bursts. In late April, CHIME picked up a signal that looked like a fast radio burst, coming from a blazing magnetar some 30,000 light-years from Earth. If the signal is confirmed, it would be the first FRB detected in our own galaxy, as well as the most compelling evidence of magnetars as the source of these mysterious cosmic sparks.