Discovered the Closest Radio Burst to Earth

An international study, which also involved the three Italian INAF radio telescopes, discovered a fast radio burst (FRB) that exploded in an unexpected way in an anomalous region and closer to Earth than expected. The contribution of the INAF team in Cagliari was decisive.

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In two articles published today in the journals Nature and Nature Astronomy, an international team of astronomers presents observations that bring scientists closer to solving the mystery of fast radio bursts, while also raising new questions on the subject. Some researchers from Chalmers University of Technology, ASTRON, and the University of Amsterdam coordinated the international team of scientists that located a fast radio burst (FRB), with repeating signals, within a globular cluster in the nearby galaxy M81. The discovery is unexpected because FRBs are usually found around young, massive stars, much larger than the Sun, and in regions with intense star formation activity; in contrast, globular clusters contain large quantities of ancient stars. The results of the observations, conducted as part of the European PRECISE project, confirm that this source is the closest ever spotted: 40 times closer to Earth than any other known FRB. Among the authors are several researchers from the National Institute for Astrophysics (INAF) such as Marcello Giroletti, Andrea Possenti, Marta Burgay, Alessandro Corongiu, and Gabriele Surcis.

National Institute for Astrophysics (INAF)

Fast radio bursts are unpredictable, extremely brief, and mysterious phenomena. Discovered by radio astronomers in 2007 and rarely spotted, their origin is still unknown. These events produce intense radio wave emissions for only a few milliseconds before fading away. They can release such an amount of energy that makes them visible even at cosmological distances: each FRB releases energy comparable to what the Sun emits in a day. Subsequently, the so-called repeating FRBs were also discovered, meaning fast radio bursts that repeat their emission even months after their first observation. So far, FRBs have only been seen by radio telescopes.

 

“Our observations have extremely high spatial and temporal precision, so we have understood many new and unexpected things about this phenomenon,” explains Marcello Giroletti, from INAF in Bologna, one of the authors of both articles. “The most surprising novelty is that we have located this event within a globular cluster, practically a homogeneous and very old group of stars, whereas most models for explaining FRBs seemed to suggest that these phenomena were associated with the presence of young stars.”

Globular clusters are extraordinary objects: they are very dense star systems, characterized by millions of stars distributed in a spherical configuration extending on average about 100 light-years. These objects are ancient as their formation dates back over 13 billion years.

 

Fast radio bursts are “spotted” at enormous distances from Earth, in galaxies billions of light-years away from us. The data collected by the network of radio telescopes confirm, however, that FRB 20200120E is by far the closest extragalactic fast radio burst to Earth ever observed: it is located “just” 12 million light-years away from us.

 

The source of this repeating FRB was discovered in January 2020 in the Bode’s Galaxy (M81 or NGC 3031) in the direction of the constellation Ursa Major. To study the source at the highest possible resolution and sensitivity, scientists combined measurements from the 12 telescopes of the European VLBI Network (EVN) and were able to pinpoint the exact location of the powerful cosmic explosion. EVN measurements were supplemented with data from several other telescopes. Among the antennas involved are all the INAF radio telescopes: the radio telescopes of Medicina (Bologna) and Noto (Syracuse) in VLBI mode, and the Sardinia Radio Telescope (Cagliari) as a single dish and VLBI.

 

The discovery could radically change the way FRBs are searched for and studied. In recent years, the scientific community has followed the model that indicates magnetars (a particular class of neutron stars) as the most likely origin for repeating FRBs. Since globular clusters host old stellar populations, this association strongly challenges known models, namely those involving young magnetars formed in a core-collapse supernova. Experts propose that FRB 20200120E is, instead, a highly magnetized neutron star formed by the accretion-induced collapse of a white dwarf or the merger of compact stars in a binary system.

 

Astronomers searched for further clues by analyzing their data in detail, finding another surprise. Some flashes were shorter than expected: down to a few tens of nanoseconds. Kenzie Nimmo, a researcher at ASTRON and the University of Amsterdam and first author of the article in Nature Astronomy, comments: “This tells us they come from a tiny volume in space, smaller than a football field (perhaps just tens of meters in diameter). Some of the signals we measured are short and extremely powerful. This suggests that we are indeed seeing a magnetar, but in a place where they have not been found before.”

 

Marta Burgay (INAF in Cagliari) adds: “The result is part of a very broad project aimed at locating FRBs. The goal is twofold: on one hand, to understand how far they are (and thus determine all their intrinsic properties, not just apparent ones, first of all, luminosity) and on the other hand, to determine in what type of environment they originate, to get indications on what their progenitors might be.”

 

And Gabriele Surcis (INAF in Cagliari) continues: “It is mainly the result of a very flexible way of using the European network of radio telescopes outside ordinary sessions. Whenever a sufficient number of radio telescopes have time available, some candidates are selected and patiently observed until the fast radio burst arrives, which is usually recognized using special detectors. At that point, the signals from the entire network are combined, and the event is located with very high spatial accuracy.”

 

The involvement of INAF in this field is very important, as Giroletti confirms: “With its antennas and personnel, INAF contributes to unveiling new characteristics of phenomena unknown until a few years ago! This new way of using the network of radio telescopes required a strong dose of creativity and tenacity from the entire team. Now, in addition to continuing with this European project (there are already other FRBs we are analyzing), the idea is to manage to propose an entirely Italian version of the experiment.”

 

Future observations of this system will help determine whether the source is indeed an unusual magnetar or something else, such as a pulsar or a black hole and a star in a close orbit. “Fast radio bursts seem to give us a new and unexpected insight into how stars live and die. If true, they could have things to tell us about the life of stars throughout the universe,” concludes Franz Kirsten (first author of the article in Nature and a researcher at Chalmers and ASTRON).

 

 

For further information:

The article “A repeating fast radio burst source in a globular cluster,” by F. Kirsten et al., was published online in the journal Nature.

 

The article “Burst timescales and luminosities link young pulsars and fast radio bursts,” by K. Nimmo et al., was published online in the journal Nature Astronomy.

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Responsible: Marco Galliani, ufficiostampa@inaf.it, 3351778428