(ORDO NEWS) — At the close of 2021, a joyful event took place: the Baikal neutrino telescope and the IceCube detector in Antarctica simultaneously registered neutrinos from the same black hole. This is the first time that high-energy cosmic neutrinos from one source are recorded by two telescopes at once.
On this occasion, Naked Science will tell you where cosmic neutrinos come from, how astronomers catch them, and why this is important for exploring the universe at all.
Living under the downpour
Even the loneliest person in the world is never alone. He is always surrounded by a crowd of neutrinos – extremely light particles that do not have an electric charge.
According to the calculations of theorists, every cubic centimeter of space is filled with hundreds of relic neutrinos, formed shortly after the Big Bang. There are several orders of magnitude more of them in the Universe than there are protons or electrons. But there are enough sources of these particles in modern space.
Neutrinos are born in supernova explosions, in collisions of cosmic rays with interstellar gas and the earth’s atmosphere, in the vicinity of supermassive black holes and, of course, in stars. Every square centimeter of the earth’s surface is pierced every second by tens of billions of neutrinos generated by thermonuclear reactions in the center of the Sun.
And do not expect to take a break from the neutrino shower at night, when the star shines over the other hemisphere: neutrinos pass through the thickness of our planet easier than a hot knife through butter. By the way, the decay of uranium and thorium in the bowels of the Earth also generates a stream of ubiquitous particles.
Fortunately, neutrinos cannot be armed. These particles do not hurt a person, they ignore him (in a sense, it hurts more, admit it).
The fact is that neutrinos simply fly past atomic nuclei and electrons, almost never colliding with them. Therefore, it is not only impossible from them, but also there is no need to hide from them. But for the same reason, these “uncommunicative” particles are extremely difficult to register.
Meanwhile, astronomers really need to catch cosmic neutrinos. It is the ability to pierce huge thicknesses of matter, while remaining unchanged, that makes these invisibles an invaluable source of information. For example, a photon escaping from the center of the Sun to its surface is absorbed by matter countless times and re-emitted again.
As a result, this journey takes him about 100 thousand years (what do you know about the difficulties of emigration!), He experiences the influence of all layers of the star and turns from a gamma quantum into a particle of visible light. But neutrinos born in the same reactions pierce the thickness of the Sun, like a void, and bring direct information about what is happening in the thermonuclear furnace of the luminary.
So, in 2018, thanks to the Borexino neutrino telescope, scientists “took apart” the pp-cycle – the main chain of thermonuclear reactions in the interior of the Sun. The researchers measured the neutrino flux for each of the five neutrino-forming reactions separately and made sure that it matched the predictions of the theory. And in 2020 observers isolated from the solar neutrino flux particles formed in a side of the Sun. CNO-cycle .
By the way, the ability of neutrinos to carry information through any obstacles is of interest not only to astronomers. Thanks to these irrepressible particles, some specialists measure the amount of radioactive elements in the Earth’s mantle , while others are going to monitor whether weapons-grade plutonium is produced in some reactor.
Chasing the elusive
How do physicists catch particles that almost never interact with atoms? The point is that “almost never” means “never at all.” Sometimes a neutrino still crashes into a neutron of an atomic nucleus, and then it turns into a proton.
Because of this, the nucleus turns into the nucleus of another chemical element (next in the periodic table). The first neutrino telescopes captured just such transmutations. Now in the world there is only one operating installation of this kind – the Gallium-Germanium neutrino telescope at the Baksan Neutrino Observatory (Kabardino-Balkaria).
The Baksan neutrino receiver contains 50 tons of liquid gallium. Every day, about one atom of non-radioactive gallium-71 is converted in it into an atom of radioactive germanium-71 due to the fact that a neutrino got into the neutron of the nucleus and turned it into a proton.
Catching 30 radioactive atoms that appeared in 50 tons of matter in a month is still fun, given that there are more atoms in one glass of water than there are glasses of water in the World Ocean. Of course, patience and work and atoms will collect, but certainly not in real time.
Any cosmic flare that has generated a neutrino flux can only be learned after the fact. This means that it is impossible to integrate such a telescope into a rapid response network, when one instrument, which has discovered something interesting, automatically notifies its “colleagues” that it would be nice to visit the same region of the sky.
In this regard, as early as the 1970s, talks began about neutrino telescopes operating in real time. Here’s how they work: when a neutron under the impact of a neutrino turns into a positively charged proton, according to the law of conservation of charge, a negatively charged particle must be born. She is born, since she is obliged. This particle is a muon.
The muon, in turn, crashes into another atomic nucleus, and a reaction occurs in which a new particle is born, and so on. As a result, one energetic neutrino, which came from the depths of the Universe, generates a whole stream of charged particles.
Then the fun begins. If a charged particle moves through a transparent medium faster than light propagates in the same medium, it emits photons. This is called the Vavilov-Cherenkov effect. Yes, do not be surprised: only in a vacuum does light move at the maximum speed possible in nature.
In any other medium (be it air, water or glass) it propagates more slowly, and a sufficiently energetic particle can overtake it – and itself, thanks to the Vavilov-Cherenkov effect, become a source of light. The muons produced in atomic nuclei under the impact of neutrinos are fast enough to outrun light in water or ice (but not air). The light generated by these particles signals that a space visitor, a neutrino, has been registered.
The world’s first registration of cosmic neutrinos in this way took place in 1994 with the Russian telescope NT-200. It consisted of several clusters of photodetectors suspended by ropes in the transparent water of Lake Baikal.
The current Baikal neutrino telescope, also known as Baikal-GVD (Gigaton Volume Detector), is the successor to the NT-200. This, as a famous movie hero would say, is the same swallow, only on a completely different scale.
Today it consists of eight clusters (the first was launched in 2016). Each cluster contains eight vertical strings of 36 photodetectors each. Thus, in the entire telescope there are 8 × 8 × 36 = 2304 detectors (the NT-200 had only 200 of them). It is the largest neutrino telescope on Earth after IceCube.
By the way, about the latter. IceCube works the same way, but it uses Antarctic ice instead of water. Construction of the instrument began in 2005 and ended in 2010. The telescope has more than five thousand photodetectors covering an entire cubic kilometer of ice.
However, it is not the nominal volume of the installation that is important to astronomers, but the effective one: it is always less than the nominal one, and the specific value depends both on the design of the telescope and on the neutrino with what energy it is supposed to catch.
Both IceCube and Baikal-GVD are aimed primarily at energetic neutrinos (tens and hundreds of teraelectronvolts), which generate whole showers of secondary particles. As the head of the Baikal-GVD project, RAS Corresponding Member Grigory Domogatsky, explained to Naked Science , the effective volume of IceCube and Baikal-GVD for this task is the same: 0.4 cubic kilometers. At the same time, there are other interesting tasks for which the Baikal telescope is inferior to the Antarctic one in terms of effective volume.
However, the Baikal instrument is planned to be completed. If all goes according to plan, by the end of the 2020s its effective (not nominal!) Volume will be one cubic kilometer.
Concluding the conversation about the methods of detecting neutrinos, we note that there are other ways to catch a capricious particle. So, the above-mentioned Borexino registers collisions of neutrinos not with atomic nuclei, but with electrons (the so-called neutrino scattering by electrons). As a result of such collisions, electrons do not turn into anything, but receive additional energy. True, Borexino is designed to study low-energy neutrinos, primarily solar ones.
More fun together
On December 8, 2021, the Baikal telescope recorded a particle with an energy of 43 teraelectronvolts. Yes, just one. Even thousands of photodetectors spread over hundreds of millions of cubic meters of water or ice record only a few high-energy neutrinos per year. It’s not so easy to catch the elusive.
A few hours earlier, IceCube detected another particle with an energy of 172 teraelectronvolts. Both neutrinos came from the region of the sky where one of the brightest blazars is located (we will tell you what it is just below). And it must happen that it was at this moment that the blazar experienced the brightest flash of light and gamma radiation in the entire history of observations of it.
The outbreak was also noticed in X-rays and radio waves. “Coincidence? – scientists might ask. “We don’t think!” Of course, there is no one hundred percent certainty that both neutrinos came from this particular celestial body: neutrino telescopes do not determine the direction to the source very accurately. But the likelihood is very high.
This result became another important evidence that neutrinos of high and ultrahigh energies come from active galactic nuclei, including quasars and blazars. And for the first time such evidence was obtained on two installations at the same time (previously, only IceCube pleased us with such news).
Observation with two instruments increases the likelihood that the telescope did not “see” the neutrino (we understand that the useful signal of any scientific instrument has to be extracted from the noise, and sometimes the noise can add up to a false signal). In addition, observation from two points allows you to more accurately determine the coordinates of the source.
Probably, the two telescopes will sing a duet more than once. And when the KM3NeT instrument with the same principle of operation is completed in the Mediterranean, the duo will turn into a trio.
Hello from black holes
In 2018, scientists identified the energetic neutrino source with the TXS 0506 + 056 blazar. And two years later, Russian astronomers found that almost half of all ultrahigh-energy neutrinos (from 200 teraelectronvolts) received by the IceCube observatory in ten years came from the brightest galactic nuclei.
The active nucleus of a galaxy is a black hole weighing from millions to tens of billions of suns, surrounded by a disk of matter gradually falling on it. This disk rotates at a tremendous speed (near the event horizon – the “surface” of a black hole – it is comparable to light speed). Friction between the layers of the disc heats matter up to hundreds of millions of degrees.
Each portion of gas and dust, before falling onto a black hole, releases energy of about 10 percent of mc 2 – the total energy stored in matter. This is tens of times more than in the case of thermonuclear fusion that feeds the stars.
It is not surprising that the most active of the galactic nuclei (quasars) are the most powerful radiation sources in the Universe. It is such an attraction of unprecedented generosity that is needed to give even the lightest neutrinos the energy that much heavier protons at the Large Hadron Collider never dreamed of.
It remains to tell what a blazar is. Some quasars throw into the surrounding space jets of matter accelerated to near-light speed – jets. Blazar is a quasar located so that its jets are directed towards the Earth.
This unusual perspective opens up a lot of interesting things for observers. But whether blazars emit high-energy neutrinos more often than other quasars is a debatable question.
Astronomers have many more questions for the energetic and elusive messengers of the cosmos. What processes in the vicinity of a black hole generate neutrinos? How far from its “surface” do they flow? Are there other sources of high and ultrahigh energy neutrinos in the Universe besides active galactic nuclei? Perhaps the Baikal and Antarctic telescopes will help scientists find answers in the coming years.
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