Somewhere in the beginning of this year, Brian Metzger realizedthat he was left to himself — no incoming letters, no lessons — and what maybe, just maybe, he felt for an answer to one of the most persistent puzzles of astronomy. He became furious and tried to cling to this answer, worrying that a small mistake could spoil everything, or that someone else would put all the pieces together first. “Trying to keep up, because other people may also see it,” said Metzger, an astrophysicist at Columbia University.
Together with many other scientists around the worldMetgzer spent the last few years brainstorming, trying to understand fast radio bursts (FRB). These are millisecond flashes of intense and unexplained radio signals that spread throughout the sky, temporarily eclipsing the radio pulsars in our galaxy, despite being millions of times further. Until 2013, many astrophysicists generally doubted their existence. Over the years, scientists have provided dozens of possible explanations for what could have caused them. One catalog contains 48 separate theories.
That is, there were more theories than the events themselves, which they tried to explain.
Fast radio bursts: what is it?
The FRB theory needs two parts. There is a suspect - an astrophysical monster that can release huge amounts of energy. There is a weapon - something that transforms this energy into a bright, surprising and unusual radio signal.
Now here Metzger and his colleagues believe thatwere able to recover the crime scene. Earlier this month, they published an article on arXiv.org's preprints website, in which they sketched out a method for the emergence of fast radio bursts from explosions in areas of space dotted with dense clouds of particles and magnetic fields.
Their model prefers, but does not require, a magnetar inas a source of explosions. Magnetar is a young neutron star, which sometimes belches charged particles in the form of a huge version of the coronal mass ejection that occurs on the Sun. Each new explosion crashes into the surrounding mess. When this happens, a shock wave is born, which, in turn, emits a short laser-like flash of radio waves into the universe.
"In general terms, it definitely makes sense",says James Corden, an astrophysicist at Cornell University, adding, however, that further details need to be worked out. "I would say that this is a good horse to put on."
However, what astronomers really like isthis is what Metzger's theory generates very specific predictions about what future FRBs should look like. Therefore, these predictions can be verified by testing. The new Canadian radio telescope CHIME is expected to have between one and ten FRBs per day when it will operate at full capacity at the end of this year. During the initial tests last summer, he discovered a dozen outbreaks. The results of his work were made public in January. “I think that over the next year or so we will be able to test it very well,” says Shriharsh Tendulkar, an astrophysicist at McGill University, a member of the FRB CHIME team.
At the speed of the shock wave
Theory developed by Metgzer and his colleaguesBen Margalit and Lorenzo Sironi, based on the biggest breakthrough in the FRB case. In 2016, a team led by Laura Spitler from the Max Planck Institute for Radio Astronomy in Bonn, Germany, published its findings on the first ever repeated FRB. Previously, each of these events was single. Because of this, astronomers could not track where they were born in the sky, so they did not know anything about flares, although they suspected that they appear far beyond the limits of our galaxy. But one flash appeared after another.
And soon radio astronomers were able to detect it.origin in a small, deformed dwarf galaxy. Trying to squeeze every clue from these radio signals, they found that everything is born in a dense region of plasma captured by strong magnetic fields. They also found that the surge was surrounded by a dim, but stable radio coverage. And last November, astronomer Jason Hessels (along with Spitler and others) noticed something strange: each burst of a fraction of a second duration actually contains several wavelets that gradually shift down from higher to lower radio frequencies.
For the Metgzer team, this last tipseemed surprisingly familiar. In the 1950s, physicists studied the explosive waves of nuclear explosions in order to estimate their output power. In these models, the shock fronts of nuclear explosions absorb more gas as they expand outward. This additional weight slows down the impact, and since it slows down, the radiation emitted from the shock front shifts down in frequency due to the Doppler effect.
Metgzer thought that this blast effectmay hint at the true nature of the FRB. And suddenly, suddenly, at the beginning of January, the CHIME telescope picked up another recurring event. This time repeated radio signals showed the same downward frequency shift. “The idea originated with the first repetition,” says Metzger. "But after seeing this manifestation in the FRB, I earned double the pace."
Now Metzger, Margalit and Sironi have released theirfull model, based mainly on the explanation of inputs and outputs from the first repetition. Imagine a magnetar, a city-sized neutron star forged in a supernova a few decades before, with a boiling and billowing surface. Like the sun on a bad day, this young magnetar releases random bursts that shoot electrons, positrons, and maybe even heavy ions at speeds close to the speed of light.
When this material starts up, it encounterswith older particles emitted in previous bursts. Where a new ejection collides with an old one, it accumulates a shock shock, within which magnetic fields rage. When the shock is pushed out, the electrons inside rotate along the lines of the magnetic field, and this movement creates a burst of radio waves. As the shock shock slows down, the signal shifts from higher frequencies to lower ones. And astronomers on Earth receive exciting radio messages.
And although it all sounds interesting, the idea shouldwill pass the next stage of testing in the history of fast radio bursts. So far this is the most calculated and deeply thought out scenario. “They made the most detailed calculations and were able to give specific predictions for the observations,” says Spitler.
The Metgzer model predicts a number of specificfeatures that future FRBs should have. Firstly, all future FRBs should follow the same frequency reduction. They can show gamma-ray or x-ray emissions that astronomers like Spitler have already begun to search for. They must be located in galaxies, in which many new stars are formed and fresh magnetars appear. And when they do repeat, they should take breaks after astronomers observe a large flash. At this point, the system is so clogged with material that subsequent flashes can not escape.
Now the Metgzer model faces a lotother still viable theories. Fast radio bursts may be due to the merging of neutron stars, which were first caught by telescopes and gravitational wave detectors in 2017. Neutron stars can give rise to FRB, colliding with other objects, such as black holes and white dwarfs, when they themselves collapse into black holes or when their magnetic lines of force are pulled out by strong plasma flows.
And it is not clear whether all the FRB appear from the same type of event.
Data continues to accumulate, the field narrows. Over the past five months, while CHIME was in the commissioning stage, scientists discovered more bursts that they have not yet presented to the public.
After several years of studying the scattered data and theoretical dreams, the solution finally turned out to be at arm's length.
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