RadioAstron 5 years: major achievements

On July 18, 2011, the Russian scientific spacecraft Spektr-R went into orbit, which became the basis of the most successful Russian astrophysical project RadioAstron. On the occasion of the anniversary, we talked to the head of the laboratory of extragalactic radio astronomy of the Astro Space Center of the P. N. Lebedev Physical Institute of the Russian Academy of Sciences Yuri Kovalev. The conversation turned out to be informative and voluminous, but if you really want to know about the successes and results of the project, where Russian space science is leading in the world, then you should read it completely.


Yury Kovalev, photo (c) polit.ru

In the first part of the interview, we talked about the results achieved by RadioAstronom to date, and discussed near prospects.
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The RadioAstron project is a scientific experiment based on the method of radiointerferometry with super-long bases (VLBI). The essence of this method is that two or more radio telescopes work as elements of a single large system. This allows observations with a very high angular resolution, which depends on how far apart the telescopes are. With ground-based observations, it is impossible to increase the size of the VLBI network above the diameter of planet Earth. Increasing the resolution at a fixed wavelength is only possible by putting at least one of the telescopes into space. This was the telescope and became the Russian "Spectr-R". The limitation of this method is that only very bright radiation sources can be observed this way: quasars, pulsars, masers, etc. The distance (vector) between ground-based and space radio telescopes is called the base of the interferometer. Its projection on a plane perpendicular to the line of sight determines the angular resolution and is considered, for convenience, in the diameters of the Earth. The larger the projection, the higher the resolution.


(c) Roscosmos

- Yuri, I must congratulate you on the fifth anniversary, a serious age, soon to school. My first question is obvious: what would you call the most significant results and the most important discoveries of the RadioAstron project over the past five years?

- Yes, this is an expected question. And, as I understand it, this is a scientific question, although outstanding technological results have been achieved. We talked about technology on the first anniversary of the flight, and now I will talk in detail about the scientific ones. But still I would like to stress in the beginning of the conversation that no scientific achievements would have been possible if it were not for the will, the titanic efforts and faith in the victory of the project manager Academician Nikolai Semenovich Kardashev, the work of an enormous number of scientists, engineers in our country and abroad: NGO im SA Lavochkina, ASC LPI, OKB Mars, ZAO Vremya Ch (Nizhny Novgorod) and many, many others. Without them, our conversation today would not take place.


"Spectr-R" in the assembly shop (c) NPO Lavochkina / Roskosmos

Now, with regard to the results, I would single out a few key achievements, which can now be confidently spoken about. To begin with, what is closest to me, and what we started talking about after the first year of operation of the radio telescope. At first, carefully, then more confidently, now we can already talk about statistical significance based on a review of two hundred nuclei of active galaxies ( quasars ). We can say unequivocally that our understanding of the brightness level of the radiation of their nuclei has changed by an order of magnitude. They turned out to be brighter at least ten times compared to what was previously thought. We have been discussing this issue for several years now, and so far there is no distinct physical explanation that would satisfy the majority of scientists. Many are happy with this discovery, congratulating the project team, but at the same time they are “scratching their turnips” trying to explain this result, including ourselves.

Let me remind you that the problem lies in the fact that there is a theoretical substantiation of the mechanism of radiation of quasar nuclei, accelerating relativistic electrons to speeds close to the speed of light. It is believed that these electrons emit an incoherent synchrotron mechanism . Any charged particle in a magnetic field will emit a so-called synchrotron mechanism when it moves at a relativistic speed. For this case there is a prediction: it is impossible to “generate” the brightness above a certain limit, which is called the “limit for the Compton catastrophe”. It is connected with the fact that if the radiation brightness level exceeds the threshold value, the electrons will begin to actively transfer their energy to the photons. An electron flies and radiates a radio photon by this very incoherent synchrotron mechanism, and if the brightness exceeds a certain threshold, then a large number of collisions between electrons and photons occur. Electrons give their energy to photons. The photons, as a result, jump from radio to x-ray or gamma. This is called Compton backscattering. If the predicted brightness limit is exceeded, this process will be disastrous, i.e. very fast and very efficient. This means that we would observe a flare in a quasar, during which the energy was exhausted within a few hours or several days, and the brightness of the quasar would again go under the discussed limit. It was a prediction that we now see regularly broken.


Quasar PG 0052 + 251 shooting of the telescope Hubble (c) Hubble / NASA

Something is wrong. Probably, relativistic protons radiate. Many scientists who are now engaged in the subject of cosmic rays, are very interested in this opportunity. If relativistic protons emit, this means that active galactic nuclei can accelerate protons to relativistic speeds, and protons, I recall, are two thousand times heavier than electrons. Perhaps there are other mechanisms that could explain this. In any case, a consistent picture of our understanding of the nature of the radiation of quasar nuclei crumbled.

This is the first interesting result that we got almost immediately. It was believed that if the limit for the inverse Compton is not violated, then we could register the radiation of just a few quasars in the sky at the long ground-space bases of RadioAstron. Since we have a very high angular resolution and we see only the brightest and most compact details of the objects under study. Today, we have not only successfully observed, but also measured significant signals for more than one hundred and fifty nuclei of active galaxies. This is an absolutely amazing result that few people believed in, turning our ideas about quasars, which have been preserved for forty years. No previous ground-based experiments, including interferometric, as well as ground-space, for example, with the Japanese VSOP satellite, did not show a systematic violation of this limit.

The second result initially did not even belong to the key scientific program of RadioAstron. I am talking about the discovery of the scattering substructure. No one expected that such a phenomenon exists at all. We were going to study the scattering of pulsar radiation. Pulsars are “dead stars” about 20 km in size, neutron stars with extreme values ​​of magnetic fields and substance density. They manage to accelerate electrons to relativistic velocities in a magnetic field, and the field is so high that the synchrotron radiation in this case is coherent.

- I understand correctly that a pulsar is a neutron star that hits a relativistic jet towards the Earth?


Vela Pulsar (c) Chandra / NASA

- Absolutely. It should be clarified that this jet is a stream of electrons, which moves along the magnetic field lines on the magnetic poles of the pulsar. Observing the pulsars with the help of RadioAstron we hoped to find the so-called. “The scattering circle”, but it turned out that we see not just a spot, but a multitude of small specks against the background of this circle.

We generally did not expect to see pulsars on large projections of the base. Pulsars emit on long-wave radio waves, and they dissipate very effectively. RadioAstron is sensitive only to a compact structure. But it turned out that we observe bright pulsars on any projection of the base, and the size of this base practically does not affect the data obtained. Now we have already realized this effect of the scattering substructure: against the background of a blurred spot from a source of radiation that RadioAstron doesn’t actually see, but only these “specks” that interfere with radiation from multiple images of a pulsar passing through many turbulent bunches in interstellar medium, observe. This is such an explanation “on the fingers”, but in physics the effect is insanely complex.

This turned out to be important for interpreting the results not only of RadioAstron, but also for other projects, for example, Event Horizon Telescope. Now the scientists from this project have to make reservations in their research “We see either compact structures with a size of several Schwarzschild radii in the center of the Galaxy, or it is a substructure of the scattering spot”. Ground-based interferometers that work on objects with strong scattering in the center of our Galaxy can also see this effect.

Moreover, just a couple of days ago, a colleague wrote to me, who said that he was completing the development of a specific interferometric data processing technique, which allows us to restore the true image of an object hidden from us behind this scattering cloud, by restoring information about the characteristics of the interstellar medium. That is, this discovery allows not only to estimate the parameters of the interstellar medium, but also, using data on the scattering substructure, to determine the appearance of the object hiding behind it.

This result is especially pleasant for us, since we did not expect it at all. Here a well-known approach worked out that if you built a telescope, improving one of its key parameters by an order of magnitude compared to the previous ones, then you can discover something fundamentally new. And it is not at all necessary that you will know in advance what it is.

- I wanted to clarify, if you say that the effect is observed by instruments from the Earth, why did nobody pay attention to it? Did they perceive scattering as a simple spot?

- Absolutely right. We observed the pulsars, observed the center of our galaxy in the form of a large spot. And scattering was considered classic. The angular size of the spot is directly proportional to the wavelength at which the observation was carried out, to a degree of about two. The results of RadioAstron allowed to correct the method of observation by ground-based radio telescopes and we saw this substructure both in space and on Earth. Before us, for decades, the center of the galaxy was observed, and no one was able to register this effect. I note, for pulsars and quasars, we still need a ground-space interferometer.


Simulated images of a single source showing the effects of the refractive substructure at wavelengths of 18, 6 and 1.3 cm. © Johnson et al. (2016)

- Let's hope, now they with your methodology will be able to look beyond this cloud in the center of the Galaxy.

- It is necessary to say not “they”, but “you”. We conducted observations of the center of the Galaxy with RadioAstron in September 2015. Initially, such observations were not planned, since it was expected that the scattering spot would not allow to see anything. Now we have looked and, we hope, we will manage to restore the internal structure. This is a very difficult task, involving extremely difficult processing. This method will be applied for the first time in history, and we must check and double-check the results.


The visible radio image of the center of our Galaxy (Sgr A *) at a wavelength of 1.3 cm (s) VLBA + GBT / Gwinn et al. (ApJL, 2014)

We have now discussed two results. The third result is related to quasar mapping. It should be understood that RadioAstron was not initially optimized as a mapping machine. Although our mapping is done and mastered well. Not every day, we can conduct appropriate observations once every nine days, i.e. once a turn around the earth. When the satellite approaches the Earth, in the perigee area, we have the opportunity to observe space objects on both small and large base projections, when the parameters of the ground-space interferometer change rapidly over time. According to the results, we get a lot of interesting results on the study of the structure of the jets of distant quasars ...

I would highlight the following:
First, we were able to closely approach the detection and study in detail of shock waves in jets of quasars, both far from the central machine (a super-massive black hole) and near. Moreover, we see very bright radiation ... For the first time, we saw this high brightness not only near the central machine, but also far from it. This is most likely due to the interaction between the plasma of relativistic jets and the interstellar medium of quasars. We also see shock waves near the central machine, where the injection of ultra-relativistic particles into the base of the jet takes place. Previously, these sites were zamyty due to insufficient resolution.


Comparison of the survey results of the 3C84 quasar with terrestrial radio telescopes and with the help of RadioAstron. (c) VLBA and VLA / NRAO, AUI; RadioAstron / Savolainen et al.

Further, thanks to polarization measurements, we are able to restore the structure of the magnetic field at the base of the jets. And, as you understand, without knowing the structure of the magnetic field, we will not say anything about the mechanism of acceleration and the formation of relativistic jets. We see indications of the spiral structure of the magnetic field in the bases of the quasar jets.

In addition, in many cases we managed to do something completely impossible earlier from Earth - to resolve, to consider the structure of these emissions across. We began to clearly see the effects of the spread of plasma instabilities in jets. Now the difference between ground and space resolution is 5-10 times, and one of the groups made a very clear picture, which can be seen below. The ground-based result of observations gives a wide continuous stream running straight, without any special interesting details. If we use the RadioAstron resolution, it turns out that we see a quasi-spiral or periodic double structure inside these jets. Most likely, this is the result of the spread of plasma instabilities. And now, probably for the first time, we can directly compare the results of the experiment with those predicted by models from magnetic hydrodynamic modeling of relativistic jets.


The results of observations of quasar 0836 + 710 at the shortest wavelength of the RadioAstron interferometer 1.3 cm. The ground map is shown in color, and the blueprints show the results of RadioAstron. At the bottom left - the radiation pattern of the ground-based and terrestrial-space interferometers, characterizing the difference in their angular resolution.
(c) The Ground VLBI Network and RadioAstron / Vega-Garcia et al.

In addition, we see indications of the so-called stratification of the plasma flow. It seems that there is no radiation in the middle of the jet, or it is very weak. But only the edges of the quasar jets radiate. Most likely this is due to the stratification of the plasma flow. Namely, this is the result of the fact that in the middle of the quasar jets the plasma moves with a much higher speed than at the edges. Accordingly, we observe the effect of relativistic aberration: the faster the relativistic plasma moves, the more the beam of its radiation. This ray simply misses the observer on Earth, therefore from the Earth one can only see the wider radiation of the slowly moving edges of the quasar jets. This has a tremendous way to understand the nature of jets. It turns out, observing from the Earth, for a significant number of objects, we see not the entire jet as it is, but only its edges. Accordingly, the assumptions about a uniform plasma flow in jets of quasars, which were previously laid in simulation, in the interpretation of jet physics, may turn out to be erroneous.


The base of the jet quasar 3C84 (c) RadioAstron / Savolainen et al.

Now, the fourth result. It concerns the study of the so-called. cosmic masers .
Cosmic masers enhance the brightness of radio emission due to the induced emission of resonant photons by excited molecules of the medium. In our galaxy, masers are regions of the formation of stars and planets. Extragalactic masers (megamasers) are found in accretion disks of galaxies.


Star-forming regions in the center of the Arp 220 galaxy (c) Hubble / NASA

To date, we have managed to detect a significant amount of these water vapor masers on bases up to 11 Earth diameters! We see that maser formations are ultra-compact, which makes a significant contribution to the physical modeling of the processes that occur in them.

- I would like to clarify, is this maser radiation, is it directed in all directions?

“It is directed in the same direction as the maser-inducing radiation.”

- Is there anything else you can talk about? ..

- There is another area of ​​research, which is too early to make the list of results. A theme in which we invest an enormous amount of time and effort, and which promises to “shoot” if we manage to resolve all technical and technological difficulties. This topic is the measurement of the so-called gravitational redshift. If beautiful words, this is a test whether Einstein was right.

There are predictions of the general theory of relativity about how high-precision clocks should go when moving in a changing gravitational field. We have a highly stable clock on board our satellite, which flies around the Earth. We conduct special sessions to measure the so-called gravitational redshift. That is, in fact, this is the measurement of the difference of the clock on Earth and in space, and the comparison of this difference with the predictions of the general theory of relativity. This project does not yet have a result that would overlap in accuracy, say Gravity Probe A. However, there is hope that the scientific group will succeed in the near future.

In the next part, we will find out whether there are prospects for the RadioAstron scientific group for the Nobel Prize, how data from radio telescopes are technically carried out, and what promising areas of radio astronomy research are developing in Russia.