Fast radio bursts were a mystery when they were first observed. At first, every FRB followed the same pattern: a massive surge of energy at radio wavelengths that lasted less than a second—then the burst disappeared, never to be repeated. We initially suspected that the FRBs might be a hardware malfunction in our detectors, but over time, the frequency of the streams convinced us that they were real.
Since then, we have identified Sources of frequent bursts and linking the FRBs to a source that produces energy outside the radio band. This eventually helped us point the finger from one source: magnetars, or neutron stars that have very intense magnetic fields.
Now, reality is gone and the monkey key is thrown into this beautiful and simple interpretation. A new repeating source for FRBs has been identified, and it’s in a place where we wouldn’t expect to find any magnets. This does not mean that the source no of a magnetar, but we have to resort to some unusual explanations for its formation.
A magnetar is a form of a neutron star, which is what remains after a star that is massive enough to generate a supernova but not massive enough to form a black hole has collapsed. When this remnant is compressed into a soup of neutrons, the neutron star’s material shrinks until it is only about 20 kilometers wide. This compact object inherits all the rotational energy of its parent star, causing it to rotate at a rapid rate, often enhanced by the addition of matter falling from its environment.
In many cases, this rapid rotation results in pulsars, which are neutron stars with sources of radiation that appear to flash rapidly as the star rotates. In others, the neutron star ends up with an intense magnetic field, making it a magnetar. The magnetar’s intense magnetic field lines are moved by its rotation, often resulting in high-energy interactions with its environment.
But these high-energy phenomena don’t tend to last long, at least from an astronomical point of view. All of these energetic interactions with the environment cause the neutron star to shed energy, slowing its rotation and reducing the intensity of any light it produces. For example, magnetic trains are thought to have life spans of only 10,000 years before fading out into a quieter existence.
In addition, the supernova that forms magnetar stars occurs in relatively young stars, typically only a few million years old.
This combination – an early star death and a short magnetic life – means we only expect to see magnetars in regions with an abundance of young stars. It was assumed that older star clusters had seen the formation of magnetars and faded billions of years ago.
Where was that from?
The new work, by a large international team, included the follow-up on the discovery of another repeating FRB source, called FRB 20200120E. To determine the whereabouts of FRB 20200120E, the team turned to the analysis power of the European Very Long Interferometric Network, which can use up to 22 telescopes. spread all over the world. The team was able to get enough of those telescopes pointed at the repeating source to image five individual FRBs.
The way the data reconstruction from these different telescopes works, a single splash won’t give us an exact location. Alternatively, a set of potential sites can be identified. By combining the sites that correspond to each of these bursts, the researchers were able to provide a potential location for the FRB source.
This source turned out to be a globular cluster of stars in the nearby galaxy M81. Based on the remaining uncertainty regarding the location of FRB 20200120E and the frequency of globular clusters within M81, the research team estimates that the odds of not having FRB 20200120E in this globular cluster are about 1 in 10,000.
A search of this site has not revealed a consistent source of radio signals. No sources of high energy were found, based on searches with X-ray and gamma-ray telescopes. Therefore, there is no clear high-energy object there.
What is old and new again?
This site is strange. Most characteristic of globular clusters is that they consist of groups of ancient stars. It is unlikely that there has been any supernova in which neutron stars have formed for billions of years. So this would probably rule out the presence of magnets, right?
Not completely. A few mechanisms can produce a magnetar either without a supernova or long after it has occurred. These mechanisms are mostly dependent on a nearby companion star. If the companion was an ordinary star, it could feed matter into a white dwarf star until the dwarf collapses into a neutron star. Or different groups of white dwarfs and neutron stars can fuse, also producing a neutron star. Finally, we know that an ordinary companion can “spin” a previously quiescent neutron star by feeding it matter.
Any of these processes could produce a magnetar within a group of ancient stars. It can be difficult to determine which process – if any – actually took place in FRB 20200120E, given the apparent absence of any non-burst activity from the site.
In any case, the results suggest that if magnetism is the source of all FRBs, we might expect to see them in a much wider range of environments than expected prior to this discovery. We may not wish to exclude consideration of non-magnetic sources yet.
Nature, 2022. DOI: 10.1038 / s41586-021-04354-w (About DOIs).
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