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The mystery of extragalactic radio bursts grows


Astronomer Rafael Bachiller discovers in this series the most spectacular phenomena of the Cosmos. Topics of throbbing research, astronomical adventures and scientific novelties

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Astronomer Rafael Bachiller discovers in this series the most spectacular phenomena of the Cosmos. Topics of throbbing research, astronomical adventures and scientific news about the Universe analyzed in depth.

Rapid radio bursts (FRBs) bring astronomers upside down. As the findings occur, more information about their characteristics is obtained. But its origin remains an enigma.

Bright flashes

The discovery of ultrafast bursts of radio waves ('Fast Radio Bursts' or simply FRBs) reminds us in many ways of discovering pulsars, one of the most exciting episodes in the history of astronomy. And it is that the regular repetition of the pulses led, in both cases, to evoke the possibility that they were emitted by some extraterrestrial intelligent system. In the case of pulsars, emissions related very soon to neutron stars. However, determining the origin of FRBs is taking much longer .

Recreation of a rapid burst in radio waves.

The problem has brought astronomers upside down since the first burst was detected in a data file collected in 2001 by the Parkes radio telescope (Australia). Many others followed this discovery until 2018, almost all were sporadic phenomena, but there was one of them (the so-called FRB121102) that turned out to be repetitive. These bursts or FRBs are flashes in broadband radio waves (the first ones were detected around 1400 MHz), have a very short duration (a few milliseconds) and are very bright: in those milliseconds as much energy as the Sun can radiate in one day. The positions they come from are very localized points of space, points that have recently been identified in external galaxies for five of them.

The location problem

The study of FRBs accelerated with the launch of a new radio telescope in Canada in 2018: the so-called CHIME ( Canadian Hydrogen Intensity Mapping Experiment ). Although CHIME was built primarily to carry out cosmology studies, it was soon revealed as a very suitable tool for, using the broadband of its receptors, to detect new FRBs. Shortly after its launch, it had already detected numerous bursts FRBs (more than 100 are known today). And more importantly, in 2018 it detected 8 new repetitive flashes that came to join FRB121102.

We see then that FRBs can be classified as sporadic, which have only occurred once, and repetitive: those that have produced several flashes from exactly the same place in space. We still don't know if sporadic FRBs have the same origin as repetitive ones.

The European Network of VLBI EVN.EVN / JIVE / P. BOVEN

The biggest problem for the study of FRBs is that their position is usually known with very little precision (something inherent in classical observations of radio waves). So it is not possible to identify in the sky from which precise place it comes from, from which star or galaxy. The FRB is so fast that it does not give time to track the area where it originated with another more precise telescope, for example, a network of telescopes such as the European VLBI Network ( EVN ). Thus, of the hundred known FRBs, it has only been possible to establish their origin in specific galaxies for four of them, the statistics are really poor.

Increase diversity

The first three sporadic that were detected come from very distant galaxies (over distances of more than 50,000 million light years) and massive with little star formation, while FRB121102 comes from a small dwarf galaxy with an active nucleus and great activity in formation of stars. This led to think that perhaps sporadic FRBs came from catastrophic events in very massive and distant galaxies, while repetitive ones could come from another type of star in galaxies with star formation. However, a new discovery has come to ruin this (precarious) classification.

The Spanish astrophysicist Benito Marcote, working at the European Joint Institute for VLBI ( JIVE ) in the Netherlands, has coordinated the team of astronomers who has studied in detail one of the repetitive pulses detected with CHIME: the so-called FRB180916.

Location of FRB180916 in its spiral galaxy MARCOTE ET AL. / NATURE

As El Mundo promptly reported, Marcote used the European VLBI Network ( EVN , the network in which the IGN radio telescope in Yebes participates) to measure very precisely the position of the point from which the bursts arise. Then, with the large GEMINI North telescope located in Hawaii, it was identified that this point is located in a star-forming region of a spiral galaxy just 500 million light years away. This galaxy therefore does not resemble the three galaxies that house the sporadic FRBs identified to date, nor the irregular dwarf in which the repetitive FRB121102 originates. Therefore, Marcote concludes that repetitive FRBs can have different characteristics and originate in galaxies of different types. The results of Marcote et al. Were recently published in the journal Nature .

Multiple theories

Meanwhile, CHIME continues to detect more and more FRBs of all types. In a new publication the Canadian team has just announced that it has detected nine other repetitive FRBs in the year 2019, so we already have a total of eighteen to try to locate its precise origin. And what is most impressive, in a footnote on page 3 in this manuscript, the authors announce the imminent publication of a catalog of 700 sporadic FRBs. With such a set of observations, and with the use of VLBI observations to measure the precise position of some of the FRBs, they begin to have sufficient data to discriminate between the theories that try to explain the origin of these mysterious phenomena.

Recreation of a repetitive FRB created in a massive star system.

For sporadic FRBs, the theories that have the greatest acceptance are those that refer to a great cataclysm, such as the collision of two extremely dense objects, neutron stars or black holes. For the appellants, we think of phenomena of instability in equally massive objects, perhaps also in pairs. Nor is it excluded today that many of those considered sporadic could be really recurring, but that no other bursts were detected in them due to lack of sensitivity in the observations.

The fact is that the uncertainty is enormous and the number of theories that try to explain the origin of the FRBs continues to multiply. A recent article, which can be consulted here , is continuously updated with a summary of all these theories. There is even a wiki page where they are cataloged.

With such a variety of physical theories to choose from, the hypothesis of little green little men throwing such bursts seems superfluous, at least for the moment ...

Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain .

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