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In this paper, we present the results of the second census of pulsars in the decametre wave range at the Ukrainian T-shaped Radio telescope, second modification (UTR-2). Over the past 10 yr, the worldwide number of discovered nearby pulsars has doubled, which means that there is some urgency to search for a low-frequency radio emission from these newly discovered sources. To increase the sensitivity of this census, the integration time was doubled compared with the first census of 2010–2013. As a result, the decametre radio emission of 20 pulsars was detected, and their flux densities and the shape of their pulses were obtained. The dispersion measures for 10 pulsars and the rotation periods for eight pulsars were refined. For several pulsars, the scattering time constant and FWHM were estimated in the decametre wave range. The upper limits of flux densities for 102 pulsars that have not yet been detected were also estimated.

Purpose: The analytical review of the main results of research in the new direction of the low-frequency radio astronomy, the interstellar medium radio spectroscopy at decameter waves, which had led to astrophysical discovery, recording of the radio recombination lines in absorption for highly excited states of interstellar carbon atoms (more than 600). Design/methodology/approach: The UTR-2 world-largest broadband radio telescope of decameter waves optimally connected with the digital correlation spectrum analyzers has been used. Continuous modernization of antenna system and devices allowed increasing the analysis band from 100 kHzto 24 MHz and a number of channels from 32 to 8192. The radio telescope and receiving equipment with appropriate software allowed to have a long efficient integration time enough for a large line series simultaneously with high resolution, noise immunity and relative sensitivity. Findings: A new type of interstellar spectral lines has been discovered and studied, the interstellar carbon radio recombination lines in absorption for the record high excited atoms with principal quantum numbers greater than 1000. The line parameters (intensity, shape, width, radial velocity) and their relation ship with the interstellar medium physical parameters have been determined. The temperature of line forming regions is about 100 K, the electron concentration up to 0.1 cm–3 and the size of a line forming region is about 10 pc. For the first time, radio recombination lines were observed in absorption. They have significant broadening and are amplified by the dielectronic-like recombination mechanism and are also the lowest frequency lines in atomic spectroscopy. Conclusions: The detected low-frequency carbon radio recombination lines and their observations have become a new highly effective tool for the cold partially ionized interstellar plasma diagnostics. Using them allows obtaining the information which is not available with the other astrophysical methods. For almost half a century of their research, a large amount of hardware-methodical and astrophysical results have been obtained including a record number of Galaxy objects, where there levant lines have been recorded. The domestic achievements have stimulated many theoretical and experimental studies in other countries, but the scientific achievements of Ukrainian scientists prove the best prospects for further development of this very important area of astronomical science. Key words: low-frequency radio astronomy; radio telescope; interstellar medium; radio recombination lines; carbon; hydrogen; spectral analyzer

Purpose: The results of research in continuum decameter-wave radio emission of the Galaxy background, ionized hydrogen regions, supernova remnants, extragalactic discrete sources, extended galaxies, galactic clusters, extragalactic background are given. The aim of this work is reviewing the results achieved for over 50-years of the UTR-2 radiotelescope research of our Galaxy and its population, as well as extragalactic radio sources in the continuum radio emission spectrum at extremely low frequencies for the ground based observations. Design/methodology/approach: The review, analysis, collection of archival data in various publications related to the subjectof this work. Findings: The basic results of studying the ionized hydrogen regions, supernova remnants, Galaxy background emission and its large-scale structure are given, and the maps of these sources are obtained. The catalog of extragalactic discrete radio sources of the most Northern sky part and the cosmological conclusions based on its analysis are described; the estimate of the isotropic extragalactic background brightness temperature is obtained; for the first time, the observational results for the Andromeda galaxy and two galactic clusters Coma and A2255 are given briefly. Conclusions: All the results presented here emphasize the uniqueness and importance of research in the decameter wavelength range, and the large area, flexibility of structure, continuous improvement make the UTR-2 radio telescope an indispensable tool for solving the most important tasks of modern radio astronomy, despite its respectable age. For example, only in the range of 10 to 30 MHz the ionized part of the most common element in the universe, the hydrogen, becomes optically thick and begins to absorb the synchrotron emission on the line of sight, which allows rather easy separation of thermal and non-thermal components of radioemission. This property allows to determine the ionized hydrogen regions’ electron temperature and the electron concentration on the line of sight independently in studying the hydrogen emission regions. When studying the supernova remnants, we can determine the ionized matter location by their spectrum drops ‒ before, inside or behind the remnant. Based on the HB3 supernova remnant radio imagies, an assumption was made on the existence of an ionized hydrogen relic shell aroundit, being caused by the initial ultraviolet flash of a supernova. For the first time, the maps of the Northern sky large-scale structure in the declination range from ‒15° to +85° at extremely low frequencies 10, 12.6, 14.7, 16.7, 20 and 25 MHz for the ground-based observations are published, which, besides their own scientific value, may allow to correct the UTR-2 radio telescope imaging results. Using the full-resolution UTR-2 maps and the developed method of multifrequency T‒T diagrams, it was possible to separate the background radiation into galactic and extragalactic components and construct the spectrum of the latter. From the analysis of the most complete decameter wavelength range catalog of discrete sources, it follows that there is a gap in the redshift spatial distribution for all classes of extragalactic sources. The existence of an ionized hydrogen ring in the Andromeda Nebula disk has been suggested. It is shown that the main partof the galaxy clusters decameter-wave emission comes from haloes and relics. Key words: decameter range; discrete sources; supernovaremnants; catalog; continuum radio emission; HII regions; UTR-2; background emission

Purpose: Theoretical and experimental studies of the active antenna – an element of the low-frequency radio telescope antenna array for the future observatory on the farside of the Moon. Design/methodology/approach: To study the active antenna, consisting of a complex-shaped dipole and a low-noise amplifier, we used its mathematical model in the form of a two-port network, whose electrical parameters are set by the scattering matrix, the noise parameters being set by the covariance matrix of the spectral densities of noise waves. This model allows ma[1]king the correct analysis of the signal-to-noise ratio at the active antenna output with account for the external and internal noise sources. The modelling results were compared with those of experimental measurements of antenna characteristics. A series of radio astronomy observations were made with the developed antenna under the Earth environmental conditions. Findings: A numerical analysis of the radio telescope active antenna parameters has been made in a wide frequency range of 4–40 MHz. Two versions of the low-noise amplifier were developed to operate in the active antenna under the space and Earth environmental conditions. Under the Earth conditions, it has been experimentally proven that the range of problems, which such radio telescopes can effectively solve at low frequencies, is quite wide – from the solar research to the search for cosmological effects. Conclusions: The results of numerical simulations and experimental measurements obtained in this work have shown a satisfactory agreement between them for the most of the frequency range. The results of this work can be useful in the research and development of active antennas designed for operation at the decameter and hectometer wavelength ranges, particularly those intended for using under the space environmental conditions. Keywords: active antenna, Moon, radio astronomy observations, sensitivity

Purpose: Review of investigations of cosmic sources radioemission scintillations due to interplanetary plasma irregularities made at the Institute of Radio Astronomy of the National Academy of Sciences of Ukraine, from the first observations in the mid-70s until now. Design/methodology/approach: In the course of preparation of this paper, the authors have reviewed, analyzed and summarized the information being published in the home and foreign publications, and reported at scientific conferences. Findings: The investigations of the interplanetary scintillations carried out at the Institute of Radio Astronomy, NAS Ukraine have been reviewed. A retrospective discussion has been made on how in the course of these researches the knowledge about the basic parameters of interplanetary scintillations in the decameter wavelength range, as well as that on the important parameters of the solar wind and its structure, have been obtained. Also, various methods of processing and analysis of experimental data were offered, and new means for receiving cosmic radiation were created. The place and importance of the discussed researches for the world science are shown. Conclusions: Over the years since the beginning of the research, the think tank of the Department of Low-Frequency Radio Astronomy of the Institute of Radio Astronomy, NAS Ukraine has obtained a number of new relevant results, which bring Ukraine into the cohort of world centers of interplanetary scintillation researches. The construction of a new GURT radio telescope, among other things, creates new prospects for the development of this relevant line of investigation. Key words: interplanetary scintillations; decameter wavelength range; solar wind; solar wind stream structure; coronal mass ejection

International audience; Purpose: The results of studies of sporadic radio emission of several types of space radio sources, including neutron stars and planets of the Solar System, are presented. The aim of this work is to review the latest achievements in the study of low-frequency radio emission of the Solar System planets and transient signals similar to pulsar pulses using the UTR-2 radio telescope. The importance of the development of the verification methods of the spaceborne radio emission in the study of sporadic signals from various sources is shown. Design/methodology/approach: The studies of sporadic signals of different nature are based on the common set of procedures for cleaning records from the terrestrial radio frequency interference (RFI) in the frequency-time pattern using the information on the nature of the particular type of sporadic radio emission, possible types of interference and signal distortion. Characteristic features of sporadic radio emission of different sources are given, and for each of them the optimal method of signal extraction is developed. The efficiency of the developed procedures for cleaning from noise using adjustable parameters is shown. This is done on the basis of observations of diverse types of space radio sources, such as lightnings in the atmospheres of planets, single pulses of neutron stars and sporadic radio emission of Jupiter. Findings: The developed methods of signal extraction detection in the presence of radio-interference have allowed obtaining the unique results such as: the distribution of the total intensity of single pulses of neutron stars depending on the galactic latitude; the automatic search of lightnings in the Saturn’s atmosphere in data due to the carefully elaborated RFI mitigation procedures, which made it possible to obtain the Saturn electrostatic discharge (SED) emission parameters based on the most complete set of events. Increased efficiency of the selection of Jupiter’s S-radiation signals despite the data corruption by the presence of radio-interference, which was reached due to the carefully chosen parameters of data cleaning procedures, have allowed us to detect short and intensive bursts, being the most informative for determining the physical parameters of radio emission in the area of their generation. Conclusions: The large effective area and high sensitivity of the UTR-2 radio telescope allow making the sporadic radio emission study with high temporal and frequency resolutions. Due to these factors we can apply a wide range of methods of space signals’ detection in the presence of terrestrial radio-frequency interference of natural and artificial origin. The data cleaning parameters allow accounting for the characteristic features of space signals and to obtain important and even unique scientific results. Key words: decameter wavelength range; UTR-2; sporadic radio emission; transients; lightning in the atmospheres of planets; Jupiter S-bursts

International audience; Purpose: A historical review of the experimental baselopment of low-frequency radio astronomy in Ukraine, its foundation half a century ago by an outstanding scientist S.Ya. Braude to the current state. Design/methodology/approach: The constant progress of electronic, computer and digital technologies, information and telecommunication technologies, theory and practice of antenna and receiving systems design, which introduction enriched the hardware and methodological ideology of construction and usage of the UTR-2, URAN, and GURT radio telescopes, have been used. Findings: The worldwide most effective national experimental radio astronomy means, the UTR-2, URAN, and GURT decameter-meter wave radiotelescopes, have been created and improved. The best combination of the systems main parameters: sensitivity; frequency band; spatial, frequency and temporal resolutions; noise immunity; uniformity of amplitude-frequency and space-frequency characteristics and multifunctionality has been provided. Conclusions: For the half a century of radio astronomical scientific and technical at the Institute of Radio Astronomy of NAS of Ukraine, the high astrophysical informativeness of the low-frequency radio astronomy and the possibility of creating a highly efficient experimental base – giant radio telescopes of decameter-meter wavebands have been proved. Today, the Ukrainian radio telescopes are well known and recognized world-wide being indispensable and most in demand by the scientific community. The founder of the decameter radio astronomy in Ukraine, the eminent scientist Semen Yakovych Braude was not mistaken when he decided to start radio astronomical explorations. The memory of him will always remain in the minds and hearts of many generations. Key words: low-frequency radio astronomy; radio telescope; phase shifter; antenna amplifier; digital signal recorder; effective area; sensitivity; resolution; noise immunity

Purpose: Due to the significant increase of a number of pulsars discovered with high-frequency radio telescopes over the last ten years and the necessity to obtain accurate parameters of their decameter radiation (as well as to identify single pulses detected with the UTR-2), the second decameter census of these sources with the UTR-2 radio telescope became needed. Design/methodology/approach: Single pulses detected as a result of the first decameter survey of pulsars and transient emission sources can be anomalously intense pulses of the recently discovered pulsars. Identification is only possible with an accurate information on the dispersion measure (DM) of each of these sources. Given that the new pulsars are likely to have a low radiant flux density, in contrast to the first decameter pulsar census [1], the second one plans to increase the signal-to-noiseratio by increasing the observation time of each pulsar. This paper gives a complete list of the studied sources, limited by the dispersion measure (DM< 30 pc·cm-³), period (P> 0.1 s) and declination (δ>-10º), known for the beginning of 2020, as well as some examples of the decameter pulsar radio emission detected by means of pipeline data processing with the possibility of flexible adjustment of search parameters. Findings: For the first time in the low-frequency wavelength range, the radio emission from the PSR J2325-0530, PSR J0613+3731, and PSR J1426+52 pulsars has been detected. The main result of present work is specified period for the latter pulsar (PSR J1426+52), being 0.995866 s ± 5 µs, the other parameters of its radio emission are also specified. Conclusions: High sensitivity of the UTR-2 radio telescope, its receiving equipment and efficient data processing and analysis pipeline allow detecting the decameter radiation of weak pulsars, as well as obtaining their parameters with the accuracy sufficient for identification of previously discovered transient signals.

PSR B0950+08 is a bright non-recycled pulsar whose single-pulse fluence variability is reportedly large. Based on observations at two widely separated frequencies, 55 MHz (NenuFAR) and 1.4 GHz (Westerbork Synthesis Radio Telescope), we review the properties of these single pulses. We conclude that they are more similar to ordinary pulses of radio emission than to a special kind of short and bright Giant Pulses, observed from only a handful of pulsars. We argue that temporal variation of properties of interstellar medium along the line of sight to this nearby pulsar, namely the fluctuating size of decorrelation bandwidth of diffractive scintillation makes important contribution to observed single-pulse fluence variability. We further present interesting structures in the low-frequency single-pulse spectra that resemble the "sad trombones" seen in Fast Radio Bursts (FRBs); although for PSR B0950+08 the upward frequency drift is also routinely present. We explain these spectral features with radius-to-frequency mapping, similar to the model developed by Wang et al. (2019) for FRBs. Finally, we speculate that microsecond-scale fluence variability of the general pulsar population remains poorly known, and that its further study may bring important clues about the nature of FRBs., Accepted by A&A. This version includes a number of minor corrections, including corrected FRB luminosities on the time-luminosity phase-space plot for radio pulses from neutron stars and repeating FRBs

This paper presents initial results of repeated observations of powerful signals’ sources – the first-time detection of repetitive sporadic radio emission from one of these sources at low frequencies. This and other sources were found in the survey of the Northern sky in order to find sporadic radio emission sources in decameter wave range.

This paper presents parameters of the second decameter census of pulsars at UTR-2 radio telescope. In addition, the first detection results and decameter radio emission parameters of 5 pulsars are presented.

Purpose: The abstract presentation and review of the works of the last 40 years in the field of low-frequency radio astronomy and a summary of their main scientific and technical results, which have been awarded the State Prize of Ukraine in the field of science and technology for 2018. Design methodology/approach: Unique domestic tools and methods of observation were used for complex long-term experimental and theoretical studies of a large sample of the Universe objects in the range of decameter–meter radio waves. Findings: High-informative capabilities of low-frequency (8 to 80 MHz) radio astronomy studies of the Solar System, Galaxy and Metagalaxy, which have allowed one to detect and study new astrophysical objects and phenomena, to build and refine their models and mechanisms of cosmic radio emission are shown. Conclusions: Based on the creation, modernization and use of the world-largest decameter–meter wave radio telescopes UTR-2, URAN, GURT, a large amount of priority scientific and technical results and fundamental discoveries have been obtained in recent decades. New objects and phenomena, radio emission mechanisms, previously unknown energy, spatial, frequency, time, and polarization parameters of signals, which change in an unexpectedly large range of values have been discovered. The means, methods and results of domestic radio astronomy are well known and recognized in the scientific world, they are in-dispensable, fully demanded and actively used in international studies. The presented cycle of works, the obtained results and the award of the State Prize of Ukraine in the field of science and technology confirm and develop the priority of Ukraine in one of the most relevant areas of modern astronomical science – low-frequency radio astronomy.

Purpose: Finding the possibility of pulsar radio emission studies in the meter wavelength range with a small-sized subarray of the low-frequency GURT radio telescope of new generation. Development of the joint pulsar radio emission observations technique for the meter and decameter wavelength ranges at the GURT and UTR-2 radio telescopes, respectively, for studying the radio emission properties in a wide frequency range. Design/methodology/approach: The unique location of the two radio telescopes with overlapping frequency ranges at the same observatory area, and therefore under the conditions of the same propagation medium influence on the pulsar radio emission propagation recorded with these radio telescopes allows to determine the radiation flux spectra of these sources within a wide frequency range. The pulsar pulses averaged over 2-4 h are studied here. The UTR-2 structure flexibility and simultaneous using of several radio astronomy receivers allow analyzing the signals received with the whole radio telescope in the typical additive mode and with its separate subarrays, too. Simultaneous signal recording in these configurations allows controlling the influence of ionosphere on the signal propagation within the UTR-2 frequency range. Findings: With the GURT radio telescope subarray consisting of 25 active antenna elements the emission of 16 known pulsars within the 30÷70 MHz range was detected. Jointly with the UTR-2 two sessions of simultaneous wideband observations of pulsar radio emission were made within the 16.5÷70 MHz range. In the range of 16.5÷70 MHz, the average values of flux density and dispersion measures, as well as variations of these parameters for pulsars В1133+16, В1508+55 and В1919+21 were obtained. Conclusions: The studies made confirm the possibility of effective usage of the separate GURT radio telescope subarrays for radio astronomy observations, and in particular for pulsar radio emission studies. Long-term observations of pulsars in a wide frequency range allow obtaining the average values of pulsar radio emission flux densities, their spectral indices and the pulsar dispersion measures, as well as the variations of these values.

Radio telescope UTR-2 has recently accomplished a survey of the northern sky to find pulsars and transient sources of radio emission. On the one hand, studies in such a low-frequency range are very difficult because of the extremely high influence of scattering in the interstellar medium, very strong radio interference and high temperature of Galactic background. On the other hand, large dispersion delay allows to determine the dispersion measure (DM) of broadband signals with a small characteristic length very accurately.

Ukrainian scientific community has a large experience in the study of the space by radio telescopes. In the paper, the corresponding instruments and obtained results are discussed. The focus is made on the usage of the decameter radio waves for such investigations. Novel directions of the space studies and mysteries of the Universe to be resolved are discussed as well.