Annotation of the results obtained in 2020
The scientific group is completing the work on the project, having processed new extensive observational material in the last year of its implementation and received results that will have wide further development. We carried out and processed quasi-simultaneous VLBI observations at 2 and 8 GHz for one thousand VLBI-Gaia quasars, carried out on the VLBA aperture synthesis system as part of our application. Their analysis made it possible to obtain the most massive measurements of the frequency shifts of the nuclei of active galaxies. This huge observational material will allow us to study in more detail the previously discovered nuclear shifts between radio optics (VLBI-Gaia), to estimate the misdirectionality of emissions in radio between sub-pc, pc and kpc scales, as well as its relationship with the geometry of the jet. Measurements and analysis of frequency-dependent shifts in the position of the VLBI core in 142 objects of the MOJAVE-II sample were carried out by the method of two-dimensional cross-correlation of optically thin emission regions using images reconstructed at different frequencies (5, 2, and 1.4 GHz) with the same radiation pattern size , field of view and pixel size. Estimates of the magnetic field strength in the VLBI core and at a distance of 1 pc from the true start of the jet are obtained. Two-frequency maps of the distribution of the spectral index, which characterize the energy distribution of the emitting particles, have been constructed, combining the images over the regions with the optical-thin regime of synchrotron radiation, the positions of which are achromatic. In addition to massive studies of large samples of AGNs, we paid attention to individual interesting objects. The radio galaxy M87 is perhaps the most popular and most thoroughly studied AGN today, and it would seem difficult to discover something previously unknown about this object. Nevertheless, on the basis of unique highly sensitive VLBI data at 8 and 15 GHz, the most detailed map of the spectral index of the radio emission of this object on parsec scales was obtained, which for the first time in detail demonstrates the presence of at least three components in diameter associated with the central channel and the shell. The flattening of the spectrum in these regions indicates a significant stratification of the plasma in the M87 jet. Taking into account the Faraday rotation, which we carried out, allowed us to reconstruct the picture of linear polarization of radiation, which agrees with the structure of the magnetic field expected in the framework of the MHD theory. The analysis of multifrequency multiepoch observations of quasar 0528-279 - a representative of the class of objects with a peak in the radio spectrum at frequencies of the order of 1 GHz. The source is unique in its class due to the rapid variability of the amplitude of this peak, which is uncharacteristic for such objects because of the significant size of the structure components emitting at peak frequencies. The analysis performed allows us to conclude that the reason for the variability is the high value of the Doppler amplification of the radiation of particles accelerated by a shock wave in a relativistic plasma formed upon impact on a dense cloud of the medium surrounding the jet. The reacceleration of particles in this shock wave leads to an unexpectedly high magnetic field strength at a great distance from the central engine. The physical parameters of jets in active galaxies are usually estimated indirectly, based on modeling their structure with a set of Gaussian components. We have previously shown that a physical emission model can be used to estimate parameters directly - by comparison with multifrequency VLBI data. We generalized the previously used model of a relativistic longitudinally inhomogeneous jets to flows with transverse stratification. Observational results obtained in the framework of the grant on the significant stratification of the M87 radio emission turned out to be very useful and were used by us for interpretation within the framework of a generalized model. It turned out that the effect of spectrum flattening associated with the presence of a lower energy limit for emitting particles more adequately describes the central structure with a flat spectrum than the effect of synchrotron self-absorption. At the same time, it became clear that for a complete description of the entire variety of observational information (including linear polarization), further generalization of the model to the case of global magnetic fields is necessary. We have also addressed a few additional interesting problems within the subject of the grant that were not originally planned in the application. Let us mention one of them, the results of which are selected to be included as one of the achievements in the 2020 report of the Russian Academy of Sciences. Determining the sources of astrophysical neutrinos and studying the mechanisms of their formation is extremely important for understanding high-energy processes in the Universe. Using data from the IceCube neutrino telescope, observations on international radio interferometric networks and on the Russian RATAN-600 SAO RAS, it was found that high-energy neutrinos are born in the central regions of bright blazars, that is, active galaxies with jets directed at us. Protons are accelerated along the jets, the neutrinos they produce fly in the same direction. Therefore, neutrinos from blazars are recorded on Earth. The moments of arrival of neutrinos coincide with powerful bursts of synchrotron radiation in compact jets of these objects within the central several parsecs.

 

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Annotation of the results obtained in 2019
Our scientific group successfully continues to study the frequency-dependent effect of the position shift of the radio nucleus in active galaxies. At the same time, we continue to study the nature of the significant shifts between the radio (VLBI) and optical (Gaia) position of active galaxies discovered as a result of our previous work under the grant. In the work, we still use the previously proven approach, which includes combining several methods: considering a large number of objects to obtain robust statistical conclusions; unique observations of individual objects for a detailed study of their properties; development and use of new methods for data analysis and, finally, expansion of observational material. We previously predicted that the shifts between the radio and optical position of active galaxies along the jet direction are due to the dominant contribution of the optical emission of the outflow. If so, then objects with a predominant contribution of jet optical radiation should exhibit a higher degree of integrated optical linear polarization than objects with shifts in the opposite direction. The latter, according to our interpretation, are caused by the contribution of the optical radiation of the accretion disk, which means that they should have a lower degree of optical polarization. Our analysis of a sample of quasars using data in the optical range fully confirmed our predictions. Moreover, as it turned out, the distribution of the direction of polarization relative to the direction of the jet for the shifts has a significant peak at zero deg. That is, optical data allow us to study the jet structure. This, together with the previously discovered relationship between the “radio-optics” shifts and the optical colors of active galaxies, makes such shifts an even more attractive and promising tool for mass analysis of the “disk-jet” system in these objects. From a practical point of view, such shifts must be taken into account to increase the accuracy of comparison of the inertial reference systems constructed in the radio and optical ranges. Our previously proposed method for studying the VLBI structure of collimated outflows by fitting a model of an inhomogeneous jet in the spatial frequency plane, as it turned out, allows not only to estimate the physical conditions in the jet. It was this model that initially made it possible to explain the effect of the core shift observed in radio jets, which is the subject of our interest in the Project. Being a more accurate model than the commonly used sets of Gaussian functions, the proposed method allows to detect subtle details of the structure that were previously inaccessible for detection and analysis. Thus, against the background of a model inhomogeneous jet, we managed to detect a “counter-core” feature in the counter-jet region that was previously unnoticed in the data used for the analysis of the radio galaxy NGC 315. The dependence of its properties on the frequency allows to associate this component with a sharp gradient of the external density near the SMBH (r ~ 0.1 pc), which coincides with the characteristic size of the region of formation of wide emission lines observed in this object in polarized light in the optical range. The “external” nature of the opacity that forms the “counter core”, in turn, allows to obtain an estimate of the velocity of the jet plasma (> 0.7c) in this region. An analysis of the discrepancies between the data and the “inhomogeneous ejection + counter-core” model, as well as the posterior predictive verification of the fitted Bayesian model, indicate that the conical jet form is an adequate description of the outflow geometry for the data used (8.1 - 15.4 GHz). This conclusion is confirmed by the obtained values of the physical parameters of the jet. Thus, the magnetization of the flow turns out to be close to 1, which is predicted by the theory of jets acelerrated in the phase of the parabolic outflow, which have passed to the conical flow stage, characterized by an approximately constant speed. The group not only uses new methods to analyze existing data, but also works on obtaining new extensive observational material. Thus, quasi-simultaneous VLBI observations are being carried out at 2 and 8 GHz of one thousand VLBI-Gaia quasars with VLBA in the framework of our proposal, which has been granted the highest priority. We started data reduction of first observational sessions, as well as the observations of nearly two hundred objects from the MOJAVE-II sample at 1.4, 2, and 5 GHz for further analysis on measuring the frequency-dependent shift of the VLBI core position and determining its direction with respect to the inner jet direction. Of particular note is the analysis of dual-frequency VLBI data from the radio galaxy M87 (Virgo A). And not only because of the object of study, which has become the most recognizable representative of active galaxies due to its relative proximity, but also because of the record dynamic range achieved using our data (~20 000:1). Processing such deep observations is especially complicated and requires numerous checks of the result due to the possible significant influence of instrumental effects. Using simulations, we confirmed the significance of the detected effect of flattening the emission spectrum of relativistic particles in the central channel of the jet. At the same time, our analysis showed that the flattening of the spectrum at the jet edges might be associated with different coverage of the spatial frequency plane (i.e., effective resolution) in the synthesis of images at two different frequencies.

 

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