On January 9, 2019, the journal Nature announced a great finding made by the CHIME observatory, which stands for Canadian Hydrogen Intensity Mapping Experiment. Located in southern British Columbia (Canada), it is a radio telescope of peculiar design: four cylindrical antennas 100 meters long and 20 wide. It was totally coincidental, since the instrument was – and still is – in a test period: during three weeks in the summer of 2018, CHIME detected thirteen energy emissions that were as intense as they were brief.
It was further proof of the existence of a phenomenon that keeps the astronomical community on edge: fast radio bursts, better known as FRB (acronym for Fast Radio Burst). We are talking about explosions of unknown origin, located in the area of the radio electromagnetic spectrum and lasting a few milliseconds. The significance of the CHIME discovery is that one of those superefimeric bursts was repeated six times.
This fact is of capital importance for astronomers, because it allows them to observe the mysterious object with different instruments and cast the net on it. Before, only one other repetitive blast was known: FRB 121102, detected in 2012 by the Arecibo radio telescope, in Puerto Rico, and which has already been seen 150 times. Thanks to this we know that its origin is within a small galaxy located 3000 million light years from us; When it came about, life on Earth was nothing more than a procession of boring single-celled algae. FRBs release as much energy as that emitted by the Sun in a year, although the energy that reaches our planet after traveling those distances is really negligible, less than what an ant needs to bend one of its legs.
Astronomers learned of the existence of fast radio bursts in September 2007, when PhD candidate David Narkevich re-analyzed data received six years ago by the Parkes radio telescope in southeastern Australia. Narkevich found a tremendous explosion that had lasted only five milliseconds and that came from a place located 1.5 billion light years from Earth. The amount of energy thrown into space in that very small period of time was equivalent to that produced by the two nuclear power plants in Ascó, in Tarragona, for two trillion years.
Curiously, this phenomenon had already been recorded in 2001, but astronomers attributed it to some type of terrestrial interference; They could not imagine that such a phenomenal flash had a cosmic origin. As the Australian astronomer Matthew Bailes said, “the type of activity we were looking for at these distances is very weak, whereas this was so bright that it saturated the instruments.”
What produces FRBs? No one knows for sure, but experts play with the idea that we are dealing with some kind of process involving very dense small objects, such as neutron stars or black holes. In the words of the SETI Institute astronomer Seth Shostak, “FRB 121102 is like a very loud bark from a small dog.” That is, it comes out of the interior of a dwarf galaxy, several dozen times smaller than the Milky Way, a low metallicity – that is, with a deficiency in chemical elements, except for hydrogen or helium – and where a large number of stars are being formed. As in the detective game Cluedo, these are the clues that should guide us to solve the mystery.
Among the most fanciful explanations is that of those who propose that they are signs of extraterrestrial civilizations. But what would lead them to announce their presence with such bursts of energy? For Shostak, “it’s like sinking the Titanic using a flare gun powerful enough to be seen from Mars.” Well, according to Harvard-Smithsonian Center for Astrophysics astronomers Avi Loeb and Manasvi Lingam, FRBs could be leaks from planetary-sized alien objects. These supposed emitters would not be designed for communication, but to propel ships using the principle of solar navigation: equipped with huge reflective sails, beams of radio waves would impinge on them, driving them as the wind does with our ships.
However, in little more than a decade, around sixty FRBs have been detected , scattered throughout the sky. And if we take into account that they are extremely difficult to find due to their short duration, we would face a fairly common phenomenon in the universe. Some astronomers think that they occur when a celestial object collapses similarly – but hundreds of times more intensely – to supernovae that mark the end of a star’s life. In this case, the bursts would come from stars with a greater mass than a galaxy when they become black holes that also rotate at high speed. This traumatic process lasts only between ten and twenty seconds.
Another hypothesis is that of a collision between two stellar corpses: neutron stars. These objects have a mass equivalent to twice that of our sun packed together inside a sphere ten kilometers in diameter, and they rotate about a thousand times per second. This exotic collision – it is not easy for something like this to happen – was one of the first explanations given to fast radio bursts. Its author, West Virginia University astronomer Duncan Lorimer, was the thesis advisor for the student who found the first FRB.
Obviously, these are not the only conjectures astronomers have made. In fact, they have proven to be very prolific in proposing explanatory hypotheses: there are up to 48 different ones, ranging from the merger of electrically charged black holes to their counterpart, white holes, through the evaporation of micro black holes, clouds of hypothetical particles called axions or the almost forgotten primordial cosmic strings, a type of one-dimensional structure that formed shortly after the big bang.
More plausible seems to have the proposal of Brian Metzger, from Columbia University (New York). In February 2019, Metzger published together with Ben Margalit and Lorenzo Sironi an article detailing a scenario involving plasma clouds and powerful magnetic fields. The American astronomer and his colleagues started from a well-known idea: that the engine of these explosions is a magnetar, a neutron star about twenty kilometers in diameter that has an extremely strong magnetic field. Its main characteristic is that on occasions, and for a very short period of time, it releases enormous amounts of energy. The astronomical community was excited about this theory, because it makes very specific predictions about the appearance of FRBs, and looked at the Canadian CHIME radio telescope, which was constantly detecting fast radio bursts.
Metzger and his colleagues have been able to develop their model thanks to the greatest turning point in the short history of FRBs. In 2016, a team led by Laura Spitler, from the Max Planck Institute for Radio Astronomy (Germany), published the study showing that the aforementioned FRB 121102 was a repetitive phenomenon. And that the source was outside our galaxy, but … where? The problem with radio emissions is that it is very difficult to locate their origin without the support of more precise observations in other ranges of the electromagnetic spectrum, especially in the visible.
By repeating themselves, radio astronomers were able to find the culprit: a small irregular dwarf galaxy. They also determined that it came from a dense region of plasma, trapped in intense magnetic fields. They even established that the explosion was surrounded by a weak emission of radio waves. And in November 2018, astronomer Jason Hessels revealed something even stranger: Each burst contained other secondary, almost imperceptible.
This is what put Metzger’s team on track. In the 1950s, physicists studied nuclear tests in search of a way to estimate their performance. According to the models used, the shock wave created by the big explosion swept through the atmosphere as it moved. And since it had to displace a larger mass of gas, the frequency of the radiation dropped due to the Doppler effect. Metzger began to think about this process when he learned of CHIME’s discovery, because the recurring radio source captured by the Canadian radio telescope – cataloged as FRB 180814.J0422 + 73 – showed precisely that drop.
Metzger, Margalit and Sironi’s model consists of a magnetar that occasionally ejects electrons, positrons – the antiparticles of the electron – and ions – electrically charged atoms – at a speed close to the speed of light. This material travels through space like a shock wave until it encounters an older one, coming from previous bursts, which generates intense magnetic fields. Then, the charged particles from the explosion begin to move at frantic speeds following spiral paths and launching intense radio emissions. Then, as the speed of the shock wave decreases, the radio frequencies also decrease: just what you see.
Now, all scientific observation requires quantitative prediction of what is going to happen, and this is the virtue of Metzger’s theory. The model predicts that all FRBs must verify the same frequency drop. Furthermore, they have to appear in galaxies where many new stars are forming; the most massive ones end up becoming magnetars.
Finally, and if the bursts are repeated, the model predicts that some time must pass before another flash can be seen: at that point, the system is so saturated with material that subsequent flashes are not distinguishable.
The ban has been lifted. The CHIME and the Australian Square Kilometer Array Pathfinder, a network of 36 identical parabolic antennas, have your ears pinned to the sky above FRB. And if all goes well, next year the Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) will come into operation, a series of 1,000 radio telescopes with a six-meter dish placed in a very compact configuration in the South African Karoo desert.
To conclude, last November astrophysicists opened the month with encouraging news: three independent studies published in the journal Nature confirm that a signal of this type, observed inside the Milky Way, came from the magnetar SGR 1935 + 2154, located or 30,000 light years from us.
Is the mystery about to be solved?
