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Unknown nature of cosmic radio bursts
Unknown nature of cosmic radio bursts
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One of the most mysterious cosmic phenomena is fast radio bursts. These are short, several millisecond duration radio signals of an unknown nature, resulting from the release of an enormous amount of energy. More than a decade has passed since their discovery, but astrophysicists are still trying to figure out the mechanisms of their occurrence. The researchers cite neutron stars, black holes, and even transmitters of alien civilizations as possible sources.

Mysterious signals

With fast radio bursts, in milliseconds, as much energy is released as the Sun has emitted over several tens of thousands of years. The leading hypothesis is that they are caused by catastrophic events, such as the merger of two neutron stars, a flash from the evaporation of a black hole, or the transformation of a pulsar into a black hole. For a long time, it was believed that radio bursts can occur only once, but in 2015 it was discovered that the previously recorded fast radio burst FRB 121102 is repeated in a non-periodic manner.

FRB 121102 is located in a dwarf galaxy three billion light-years from Earth, and for several years remained the only known source of recurring radio bursts, despite careful searches. However, in January 2019, an article by scientists from the Canadian CHIME collaboration appeared in the journal Nature, in which it was reported that signals from another source were re-registered - 180814.J0422 + 73. The CHIME (Canadian Hydrogen Intensity Mapping Experiment) interferometric radio telescope has recorded six fast radio bursts that came from a galaxy 1.3 billion light years away.

The signals in their frequency structure and spectral characteristics resembled the signals from FRB 121102, which indicates a similar mechanism of their formation and the same nature of the source. The discovery indicates the existence of a separate type of fast radio bursts, which cannot be caused by catastrophic events precisely because of their recurrence.

Silent galaxy

In August 2019, an international team of scientists first identified the source of a single fast radio burst FRB 180924, which originated in a galaxy four billion light-years away.

Using the ASKAP radio interferometer located in Australia, astronomers determined the location of the FRB source, and then calculated the distance to it by analyzing data from the Gemini, Keck and VLT optical ground-based telescopes. It turned out that the radio flare occurred in a massive galaxy the size of the Milky Way, 13 thousand light-years from its center. A characteristic feature of the galaxy is the absence of processes for the birth of new stars.

This is in contrast to the repetitive signal FRB 121102, which is located in the region of active star formation. Thus, single and repetitive fast radio bursts must have different origins. In the case of FRB 121102, the radio signal seemed to pass through a powerful magnetic field around a magnetar, a special type of neutron star.

Soon, astronomers at the California Institute of Technology in the United States reported the discovery of another fast radio burst FRB 190523, which also occurred in a relatively calm environment - in a galaxy that is analogous to the Milky Way and is 7.9 billion light years distant from Earth.

Both of these discoveries disprove that fast radio bursts can occur only in young dwarf galaxies that contain a large number of magnetars.

Eight twins

In August 2019, an article by the Canadian collaboration CHIME appeared in the arXiv.org preprint repository, which reported the detection of eight repeating radio signals. Two sources of radio signals - FRB 180916 and FRB 181119 - flashed more than two times (ten and three times, respectively), the rest sent repeated radio signals only once, with the longest pause between recording radio waves being 20 hours. According to the researchers, this may indicate that many FRBs are actually repetitive, but some are more active than others.

Most of the eight new fast radio bursts showed a decrease in signal frequency with each repeated burst, which may be the key to understanding the mechanism that produces these phenomena. In addition, FRB 180916 has the lowest signal dispersion rates, indicating the relative proximity of the source to Earth. It could also help determine the nature of the radio burst, the researchers concluded.

Hell stars

In late summer 2019, scientists at the National Center for Radioastrophysics in India reported that magnetars are still one of the most likely sources of fast radio bursts (at least repetitive ones).

The anomalous magnetar XTE J1810-197 was observed with the Giant Metrewave Radio Telescope. Millisecond pulses of radio emission were recorded, resembling flashes from repeating FRB 180814.J0422 + 73.

This magnetar is located 10 thousand light years from Earth. It was discovered in 2003 and gradually stopped emitting radio emissions in 2008. However, in 2018, a new outbreak occurred on it, which also gradually began to fade. Interestingly, magnetars usually do not emit radio emission, and the XTE J1810-197 was the first source of radio emission of this kind. The rarity of this object, like the repetitive radio bursts, has led scientists to believe that both phenomena may be related to each other.

Noisy hole

In September 2019, Chinese astronomers reported that they had detected new fast repetitive radio bursts (FRBs) from FRB 121102. The signals were detected with a 500-meter FAST radio telescope with a 19-beam receiver in Guizhou province. From the end of August to September, more than 100 spikes were recorded, which is a record number among all recorded FRBs.

By that time, scientists began to assume that FRB 121102 is a supermassive black hole that exceeds the mass of the Sun by 10-100 million times and generates a powerful magnetic field, and a neutron star or plasma affected by the hole could be the direct source of flares. Another possible explanation is that FRB 121102 is a magnetized plerion, a nebula fueled by the stellar wind from a pulsar.

While fast radio bursts remain unexplained so far, a lot of data was presented to the scientific community in 2019 that brings astronomers closer to the solution. It turned out that FRBs can be repeated, and they probably do it very often. In this case, they are generated by rather exotic objects like neutron stars (pulsars and magnetars), located in a suitable interstellar medium. Single bursts occur in less turbulent conditions: galaxies where star formation is very slow. Such phenomena, most likely, really occur due to catastrophic processes.

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