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Интервью Юрия Ковалева изданию Аmerican scientist

First Person: Yuri Kovalev

Brian Malow

For decades, physicists have pursued direct evidence of black holes. In 2011, Russia launched Spektr-R, or RadioAstron, a space radio telescope, to collect the confirmational data. Yuri Kovalev, a physicist with the Lebedev Physical Institute of the Russian Academy of Sciences in Moscow, is the project scientist of the RadioAstron space interferometer, which uses the interference between two beams of light to make precise measurements of distant galaxies. Kovalev discussed his research with contributing correspondent Brian Malow, during a press tour hosted by the Moscow Institute of Physics and Technology.

What is your specific area of study within astrophysics?

I’m studying active galactic nuclei, which are galaxies like ours, but located far away, billions of light years away from us, and they have a little bit bigger supermassive black hole in their centers. The idea is, in the center of our galaxy we have a black hole, we believe, of just a few million solar masses, but it is not active. The center of our galaxy is not active in the sense that it doesn’t have a lot of matter falling into it and it doesn’t have very hot plasma, which is ejected away from the center. Active galactic nuclei, called quasars, would have a larger black hole, something like a billion solar masses, and would have a big accretion disk, a disk of dust and matter, which falls on the center. About 10 percent of this matter is ejected out in the form of very narrow relativistic jets.

These jets are like two spokes coming out perpendicular to the disk?

Yes, they are. We believe they are about perpendicular and we believe that there are two. Quite often we see only one because of relativistic effects. One of them, which faces us, looks much brighter. We believe that at some periods of time the center of our galaxy is more active than at others. There is a NASA satellite, the Fermi Gamma-ray Space Telescope, which has seen bubbles in gamma rays, which might hint at that.

What exactly are you interested in studying about active galactic nuclei?

I’m interested in what happens in the center, how these jets are formed and how they are emitted. We believe that these jets consist of very fast electrons. The electrons are accelerated somehow in the centers of these galaxies and then they go away from the center at near the speed of light. We want to know what the mechanism of their emission is. Also, there is a question of whether these truly are electrons. Some scientists are even discussing that these could be protons. If these are protons, and we can check it with our project, then it is truly a very cool thing because it is 2,000 times more complicated to accelerate them. Protons are 2,000 times heavier than electrons.

Prior to the launch of RadioAstron, how did you and your colleagues study active galactic nuclei?

I was studying them using two ground-based telescopes, both in Russia. We have the largest radio telescope in the world, the RATAN-600. It is 600 meters in diameter. Later I often used telescopes all around the world, such as in the United States and Europe. When the research community formed this huge instrument, which we’ll call the interferometer, it had to be an international project.

The idea of Very Long Baseline Interferometry (VLBI) was proposed by Russian scientists, Leonid Matveenko, Nikolai Kardashev, and Gennady Sholomitskii. VLBI uses data from multiple radio telescopes to analyze signals from astronomical radio sources, such as a quasars. It was first successfully implemented by American scientists, which is often the case, and was one of the first intercontinental experiments between the United States and Russia. American scientists approached the Lebedev Physical Institute to suggest having an experiment between a telescope in Green Bank, West Virginia, and a telescope in Pushchino, a town in the southern Moscow region. They replied that it was a great idea, but suggested that we have another telescope, which is even better, in Crimea, so you are welcome to come.

It was the middle of the Cold War, truly difficult times, and the American side just came and it was wonderful that they realized that Russians are the same as Americans in many respects. Later, it was determined that the reason why it was not supported to have an experiment between America and Pushchino is because when you do the experiment, you, as a by-product, measure a position of your telescope with an accuracy of about one centimeter. So to have a reference point with an accuracy of one centimeter, near Moscow, in the middle of the Cold War, would be absolutely impossible. The experiment had to be sent to Crimea. Even in these very difficult times astronomy or astrophysics was one of the very few topics that we were able to do together. So the roots of all that we are discussing right now come from the Cold War times many years ago.

How did your project come about?

RadioAstron, which is a radio satellite combined with observatories on the ground to create the largest telescope in history, was originally conceived during the Soviet times. The first official statement about it was made by the General Secretary of the Communist Party of the Soviet Union Leonid Ilyich Brezhnev, in the early 1980s. Apparently the project went through all the difficult times together with the Soviet Union. It was built, it was almost ready to go to space in the early 1990s, and then it collapsed together with the country. Then we had to bring it all back and rebuild it starting in 2004. We launched it from Baikonur on a Russian-Ukrainian rocket in 2011, and it has been operational since early 2012. It operates under a so-called open sky policy, so any person can apply for observing with us including our fellow American colleagues, and they do.

What was the process of loading and deploying the satellite?

We produced a flower of 27 petals, which was put together on the ground and fit nicely into the 3-meter diameter of the rocket. Then, five days later, after the launch, it was unfurled, not without problems.

After launching it, the Lavochkin Research and Production Association commanded the spacecraft, the telescope, to unfurl. The ring in the base of the telescope started to rotate and pull open these petals. Ten minutes later, the ring stopped, but we on the ground didn’t get a confirmation from the detectors that the petals had unfurled successfully. Several hours of attempts didn’t change anything. Later Lavochkin estimated and realized that it was not unfurled, just by several centimeters. This would mean the end of the mission. Just several centimeters was not enough, because we had to build a parabolic structure with an accuracy of 1 millimeter.

They realized that they probably had a temperature gradient in the base of the telescope. They rotated it so that the Sun would shine on the base of the telescope and would take care of the residual temperature gradient in the base. One day later they came back; they had another telemetry session. They commanded the telescope to unfurl again, and it unfurled successfully. We got confirmations from all the sensors. Then it was fixed and it has worked ever since.

That must have been an exciting moment. Were you there?

I was watching it on a video conference that was streamed from Lavochkin. It was a Friday, in July 2011. I’ll never forget it.

What’s the mission of RadioAstron?

We are studying different kinds of objects in the sky. Number one is actually active galactic nuclei or quasars. Number two is pulsars, dead neutron stars in our galaxy. We use them to study the interstellar medium in our galaxy and the scattering of radio waves. Number three is so-called masers. These are clouds of, for example, water vapor, which are located both in our galaxy, in regions where planets and stars are born, and also they are located in other galaxies in their disks. In addition to that, there is a very peculiar and very important object for us, which is the center of our own galaxy, and it has its own story.

We are also trying to do a little bit in gravitational astronomy, because we have a very accurate atomic clock on board the satellite. Every nine days—the time it takes to complete its elliptical orbit—the clock on the satellite goes through a different gravitational potential than the clocks that are located on the ground. Because the elapsed time differs based on the observers’ distance from a gravitational mass, the clock on our satellite advances differently. Einstein’s theory of general relativity predicts how the clocks should behave, so we can check the theory by comparing its predictions with what we actually see on the satellite.

What have you learned about active galactic nuclei?

The thing with the quasars is that we thought that we understood very well the theory of how the jets in quasars emit their very bright radiation. And the theory has predicted that cores of these quasars cannot be brighter than a certain level. For example, if you inject a blob of very hot plasma, very energetic electrons, into this jet from the central supermassive black hole and its surroundings, then it will emit very brightly. However, the electrons will very quickly lose their energy due to the process of falling to a lower state.

In what is called an inverse Compton process, electrons emit photons, and because they emit very many photons, they hit other photons. There is also a higher probability that the electrons will then hit a photon, causing the electron to lose its energy. It has been predicted that, because of these collisions, the quasar cannot appear brighter than a certain limit, a result that is called a Compton catastrophe.

We have checked this theoretical prediction and we have shown that actually it is wrong. We have observed the quasars to be at least 10 times brighter than the theory predicts. This is truly exciting because there is nothing better in the life of a scientist than to show that theories are wrong.

People sometimes don’t quite appreciate that scientists would like theories to be disproven.

Let me tell you that theorists are the first to be happy about it. That is how science works. Science works by showing that the theory is wrong. And by showing it, you allow the theory to evolve and to understand nature around you better. One of the explanations is that these are not relativistic electrons, but they are relativistic protons. Because they are 2,000 times heavier, they allow you to have a much higher limit on the brightness. In principle, if these are relativistic protons, this will solve our problem. But then there is an even bigger problem, how to accelerate them. We don’t know how to accelerate protons to become relativistic—protons that move very close to the speed of light.

Have you achieved your mission goals?

We believe that we have achieved the main scientific goals of the mission. However, we collected the last round of proposals for the mission about a month ago, and the number of proposals has gone up.

The demand is even greater than before, because our initial results have demonstrated the potential of studying the universe at this extreme resolution. RadioAstron has proven to be useful for many different applications. Therefore, the mission has been officially extended by Roscosmos, the Russian space agency, until the end of 2018.

The mission is already past its expected lifetime of five years, but it will go a little longer. What’s next for you?

We truly hope to see the center of our galaxy through the interstellar fog. That is what we haven’t achieved yet. We are working together with the Event Horizon Telescope (EHT), a ground-based telescope that has been in the works since 2012. EHT is expected to produce its first image of the center of our galaxy later this year.

Talking about the long-term future, we are currently building a mission plan for the Millimetron observatory, which will detect signals at millimeter and submillimeter wavelengths. It will be similar to the Herschel Space Observatory, which was active from 2009 to 2013. The resolution will be better than what we have with RadioAstron.

Everything will be more transparent in the universe. We will have less scattering and also less absorption. This will help us see central supermassive black holes, both in the center of our galaxy, if not already observed by that time, and in other galaxies.

Источник: http://www.americanscientist.org