Fast radio bursts, or FRBs, are short, intense flashes of radio waves that are believed to be the product of small, distant and extremely dense objects, although what these objects can be has been a long-standing mystery in astrophysics. FRBs typically last a few milliseconds, during which they can eclipse entire galaxies.
Since the first FRB was observed in 2007, astronomers have documented 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 punctual, flashing briefly before disappearing completely. In a handful of examples, astronomers have observed rapid radio bursts repeatedly from the same source, but with no discernible pattern.
This new FRB source, which the team cataloged FRB 180916.J0158 + 65, is the first to produce a periodic or cyclical model of rapid radio bursts. The pattern begins with a noisy four-day window, during which the source emits bursts of random radio waves, followed by a 12-day period of radio silence.
Astronomers have observed that this pattern of 16 days of rapid radio bursts occurs repeatedly over 500 days of observations.
“This FRB we are talking about now is like a clock,” says Kiyoshi Masui, assistant professor of physics at MIT’s Kavli Institute for Astrophysics and Space Research. “This is the most definitive diagram we have seen from any of these sources. And this is a great clue we can use to start researching the physics of what is causing these lightning bolts, which no one really understands. “
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, the University of Toronto and the National Research Council of Canada, which operates and analyzes data from the 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 the details of the new observation in the review Nature (“Periodic activity from a fast radio burst source”).
A radio view
In 2017, CHIME was erected at the Federal Observatory for Radio Astrophysics in British Columbia, where it quickly began to detect rapid radioactive 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 without moving parts. Rather than rotate to focus on different parts of the sky, CHIME stares at the entire sky, using digital signal processing to locate the region of the space where the incoming radio waves come from.
From September 2018 to February 2020, CHIME selected 38 fast radio bursts from a single source, FRB 180916.J0158 + 65, which astronomers have traced in a churning region of stars on the outskirts of a huge galaxy. spiral, 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 explosions are something we have never seen before, and this is a new phenomenon in astrophysics,” says Masui.
The phenomenon behind this new extragalactic rhythm is a big unknown, although the team explores some ideas in their new article. One possibility is that periodic bursts can come from a single compact object, like a neutron star, which is both rotating and oscillating – an astrophysical phenomenon known as precession. Assuming that radio waves emanate from a fixed location on the object, if the object rotates along an axis and this axis is not pointed towards the direction of the Earth every four days out of 16, 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 emits radio waves and is in an eccentric orbit which 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 deform and burst. to move away. This pattern would repeat itself when the neutron star folds back into its orbit.
The researchers envisioned a third scenario, involving a radio source that circles a central star. If the star emits a wind or a cloud of gas, each time the source passes through the cloud, the cloud gas could periodically amplify the radio emissions from the source.
“Maybe the source always emits these bursts, but we only see them when it crosses these clouds, because the clouds act like a lens,” says Masui.
Perhaps the most exciting possibility is the idea that this new FRB, and even those that are not periodic or even repetitive, can come from magnetars – a type of neutron star with an extremely strong magnetic field. The details of the magnetars are still a bit mysterious, 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 fast radio bursts from these magnetars, and this periodicity that we have 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 can actually be a viable source for rapid radio bursts. In late April, CHIME received a signal that resembled a rapid radio burst from a flared 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 convincing evidence of magnetars as the source of these mysterious cosmic sparks.