Short but extremely high-energy radio bursts from space puzzle astronomers because the causes of these fast radio bursts are largely unknown. Now another case is causing new surprises. Unlike most of its predecessors, the radio flash captured by the CHIME radio telescope in Canada and repeated several times comes from a massive but old and inactive galaxy – there is hardly any star formation or young stars in it. This does not fit with previous theories, according to which fast radio bursts are released during the supernova of massive, short-lived stars and their transformation into magnetars. Instead, there must be at least some such radio bursts that arise in other ways. The similarity of the current discovery to an earlier “outlier” could now provide initial clues.
Since their discovery in 2007, fast radio bursts (FRBs) have puzzled astronomers. These cosmic radio pulses only last a few milliseconds, but release as much energy as our sun does in a whole day. While most of them are one-off events, there are also a few that occur in series or in sporadically repeated episodes. Astronomical observations also show that most of these radio bursts come from distant, actively star-forming galaxies and appear to originate in a strongly magnetized environment. In 2020, the first observation of a fast radio burst in our own galaxy provided the first clues to a possible originator: astronomers were able to trace this radio pulse to a magnetar near the center of the Milky Way – a rapidly rotating, strongly magnetic neutron star. “The prevailing theory is that fast radio bursts come from magnetars formed by core collapse supernovae,” explains lead author Tarraneh Eftekhari from Northwestern University. However, among the more than 600 radio flashes that have now been detected, there are also some whose characteristics do not match this original scenario.
Repeated radio bursts from an ancient, inactive galaxy
This includes FRB 20240209A, discovered in February 2024 with the Canadian Hydrogen Intensity Mapping Experiment (CHIME). This fast radio burst belongs to the rare type of “serial offender” and was repeated a total of 22 times by July 2024. Some of these repeats were also detected by smaller secondary antennas located around 60 kilometers from the CHIME. This gave Eftekhari’s team of astronomers the chance to determine the origin of these radio pulses more precisely. To do this, they also used additional observations of the suspected region of origin using spectrographs at the Keck Observatory in Hawaii. The analyzes showed that FRB 20240209A originates from a massive galaxy around two billion light-years away. “This galaxy has a stellar mass of around 100 billion solar masses. “It is the most massive host galaxy of a fast radio burst ever discovered,” report Eftekhari and his team. For comparison: the stars in our Milky Way together weigh around 60 billion solar masses.
What is surprising, however, is that the host galaxy of FRB 20240209A has almost only old stars and hardly any active star formation. “We arrive at a star formation rate of less than 0.36 solar masses per year over the last 100 million years,” report the astronomers. “The average age of the stellar population is 11.33 billion years – this is significantly older than the previously known FRB host galaxies.” In other words: This fast radio burst comes from an old, inactive collection of stars in which hardly any stars and hardly any supernovae and thus new magnetars are formed. Typically, these arise from massive and therefore rather short-lived stars. “This marks FRB 20240209A as the first repeating fast radio burst from a quiescent host galaxy,” the team writes. It is fitting that this galaxy belongs to the elliptical type. They are among the oldest forms of galaxies in the universe.
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The location of the radio flash source within this galaxy is also unexpected: the intense radio burst emanates from a location on the outer edge of the galaxy, around 130,000 light-years from its center. This is unprecedented among FRBs. “This is both surprising and exciting because fast radio bursts usually occur in the interior of galaxies, usually in star-forming regions,” says co-author Vishwangi Shah from McGill University. “The location of this FRB so far out in its home galaxy raises the question of how such energetic events can occur in regions where no new stars are forming.” In fact, FRB 20240209A is not the first radio burst from such a “galactic suburb”: In 2022, a team of astronomers discovered a fast radio burst from the spiral galaxy Messier 81, around twelve million light-years away. Further analyzes at the time showed that this Radio pulse came from an old globular cluster on the outskirts of this galaxy. However, such an environment does not fit the common scenario of a radiation burst from a young magnetar formed in a supernova.
According to the researchers, the external position and the old star population of the host galaxy of FRB 20240209A suggest that this new radio burst also comes from such a globular cluster. “In fact, it could be a twin of the M81 event,” says senior author Wen-fai Fong of Northwestern University. “This kind of ancient environment makes us rethink our standard models for FRB formation and consider more exotic formation channels – that’s exciting.” Specifically, the astronomers suspect that the interaction of white dwarfs – burned-out remnants of stars – in the dense centers of such globular clusters can generate magnetars and thus fast radio bursts. For example, a merger of two white dwarfs or a so-called accretion-induced collapse would be conceivable. This occurs when a white dwarf sucks so much material from a stellar companion that it collapses under its own gravity. “We don’t know for sure whether there is a globular cluster at the position of FRB 20240209A,” says Shah. However, an application has already been submitted for observation time at the James Webb Telescope in order to clarify this question.
“This new fast radio burst shows us that just when you think you have understood an astrophysical phenomenon, the universe surprises us,” Fong said. “This is what makes our field of research so incredibly exciting.” Eftekhari adds: “It seems clear that there are still many exciting discoveries to be made regarding fast radiobursts and that their environments could be the key to unlocking their secrets.”
Source: Tarraneh Eftekhari (Northwestern University, Evanston) et al., The Astrophysical Journal Letters, submitted; doi: 10.48550/arXiv.2410.23336