Astronomers have observed mysterious flashes in the sky from an unexpected source, a globular cluster in the galaxy M81. It is the closest source of fast radio bursts that has been located so far. The results are described in two papers to be published this week in Nature and Nature Astronomy.

Published by the editorial team, 23 February 2022

Fast radio bursts (FRBs) are unpredictable, extremely short flashes of light from space. Astronomers have struggled to understand them ever since they were first discovered in 2007. So far, they have only been seen by radio telescopes. Each flash lasts only a thousandth of a second. Yet each flash emits as much energy as the Sun gives out in a day. Every day, there are several hundred flashes across the sky. Most are located far away from Earth, in galaxies billions of light years away. Only a few have been observed so far.

In two papers published in parallel this week in the journals Nature and Nature Astronomy, an international team of astronomers presents observations that take scientists a step closer to solving the mystery, while also raising new puzzles. The team is led by Franz Kirsten (Chalmers, Sweden, and the Netherlands Institute for Radio Astronomy ASTRON, the Netherlands) and Kenzie Nimmo (ASTRON and the University of Amsterdam).

Close but surprising location

The team traced the repeating bursts to the outskirts of the nearby spiral galaxy Messier 81 (M 81), about 12 million light years from Earth. This makes it the closest source of FRBs ever found. The discovery had another surprise in store: its location corresponded exactly to the site of a globular cluster, a dense cluster of very old stars.

Extremely fast radio signals from a surprising source. A cluster of ancient stars (left) close to the spiral galaxy Messier 81 (M81) is the source of extraordinarily bright and short radio signals. The image shows in blue-white a graph of how one flash’s brightness changed over the course of only tens of microseconds. (Image: Daniëlle Futselaar/ASTRON, artsource.nl)

"It is amazing to find fast radio bursts from a globular cluster. This is a place in space where you only find old stars. Further out in the universe, fast radio bursts have been found in places where stars are much younger. This had to be something else," says Franz Kirsten.

The scientists believe that the source of the radio flashes is an object that has been predicted but never seen before: a magnetar that formed after a white dwarf star collapsed under its own weight.

Source of mysterious radio signals: an artist's impression of a magnetar in a cluster of ancient stars (in red) close to the spiral galaxy Messier 81 (M81). (Image: Daniëlle Futselaar/ASTRON, artsource.nl)

Many stars in clusters form binary stars. Some are so close together that one star attracts material from the other. Once one of the white dwarfs has absorbed enough extra mass from its companion, the star ends its life as a neutron star. "This is a rare occurence, but in a cluster of ancient stars, it is the simplest way of making fast radio bursts," says team member Mohit Bhardwaj from McGill University in Canada.

Fastest ever

To the team's surprise, some of the flashes were shorter than expected. "The flashes flickered in brightness within as little as a few tens of nanoseconds. That means they must have come from a tiny volume in space, smaller than a football field and perhaps only tens of metres across," says Kenzie Nimmo.

Future observations of the globular cluster in M81 will have to reveal whether the source is really an unusual magnetar, or something else, like an unusual pulsar or a black hole closely orbiting a massive star.

"These fast radio bursts seem to give us new and unexpected insights into how stars live and die. Like supernovas, they could tell us things about the life cycle of stars in the universe," says Nimmo.

To study the source with the highest possible resolution and sensitivity, the scientists combined measurements from 12 radio telescopes in the European VLBI network (EVN) spread halfway around the globe, including ASTRON's Westerbork Synthesis Radio Telescope and telescopes in Sweden, Latvia, Russia, Germany, Poland, Italy and China. This allowed them to pinpoint the exact location of the source of FRBs in the sky.

Papers:
The research is published in two articles in the journals Nature en Nature Astronomy.

* A repeating fast radio burst source in a globular cluster, by Franz Kirsten et al: www.nature.com/articles/s41586-021-04354-w

* Burst timescales and luminosities link young pulsars and fast radio bursts, by Kenzie Nimmo et al: https://arxiv.org/abs/2105.11446

Original press release: www.astronomie.nl

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