The structure of the radio galaxy 3C388 at decameter wavelengths

Observations of the structure of the radio galaxy 3C338 at decameter wavelengths obtained using the URAN-1 and URAN-2 radio interferometers are presented. The structure of this object at these wavelengths differs appreciably from images obtained at higher frequencies. The most probable simple models for the radio brightness distributions at 25 and 20 MHz are determined: two extended components with sizes from 40″ to 50″ whose centers are separated by 90″–100″ in position angle about 100°, and a single compact component 9″×4″ in size, whose flux density does not exceed 10% of the total flux density of the radio galaxy.     

Interferometric observations of 4C 21.53 and PSR 1937+214 at decameter wavelengths

Preliminary resuts of interferometric observations of 4C 21.53 and PSR 1937+214 at 25 and 20 MHz are presented. The observations were obtained using the URAN-1 and URAN-2 interferometers, with baselines of 42.4 and 152.3 km. In addition to the pulsar radiation, which provides about 70% of the total flux of the object, radio emission from extended components with dimensions of several tens arcseconds has been detected for the first time. The angular size of the pulsar is 3″ at 25 MHz and 4″.8 at 20 MHz. The pulsar’s low-frequency spectrum deviates appreciably from the power law derived at higher frequencies.     

Determining source angular sizes from interplanetary-scintillation observations in the saturated regime

Interplanetary-scintillation observations of the radio source B0531+194 (J0534+1927) obtained over a wide range of elongations at 111 MHz using the Big Scanning Antenna of the Lebedev Physical Institute are presented. Near the Sun, the temporal spectra of the scintillations have a two-component form, corresponding to the superposition of refractive and diffractive scintillations that is characteristic of the saturated regime. A method for estimating the angular size of the scintillating component based on measurement of the break frequency in the diffractive part of the scintillation spectrum is presented. The scintillating component as a fraction of the total flux can be determined using the maximum scintillation index. The angular size of the scintillating component in B0531+194 is found to be 0.24″ ± 0.05″, and the ratio of the fluxes in the core and halo to be roughly one-third. The flux density in the compact radio component is 5 Jy. The estimated parameters of the angular structure of the source are compared with observations at other frequencies.     

Observational studies of the angular structure of the radio galaxy 3C 234 at decameter wavelengths

An analysis of the angular structure of the radio galaxy 3C 234 at decameter wavelengths based on data obtained on the URAN-1 and URAN-2 interferometers is presented. Four of the five model components that describe the radio-brightness distribution at centimeter wavelengths are observed at decameter wavelengths: two compact components and two neighboring extended components. The fifth, undetected, component is the most extended, and encompasses the central region of the radio source, including the nucleus of the galaxy. Self-absorption is detected in the compact components, whose angular sizes are determined to be 0.27±0.03″ (northeast component) and 0.55±0.05″ (southwest component), in agreement with direct measurements at centimeter wavelengths. Most of the decameter emission of the radio galaxy is associated with its extended components.     

Deep surveys and the nonthermal noise of radio telescopes

The paper presents an analysis of catalogs of discrete radio sources and the results of deep surveys carried out with angular resolutions to 1.5″ and limiting flux densities to 9 μJy at frequencies from 80 MHz to 8.5 GHz using large radio telescopes around the world. We consider the influence on the sensitivity of a radio telescope of the nonthermal noise associated with variations in the total flux due to fluctuations in the number of unresolved sources with fluxes lower than the observed value that fall in the main lobe of the antenna beam when the direction in which the receiver is pointed is changed (the first component), and also due to sources with fluxes higher than the observed value that arrive in the scattering region of the telescope (the second component). With growth in the sensitivity and resolution of a telescope, the second component of this nonthermal noise determines to an appreciable extent the limiting capability of the telescope for carrying out deep surveys. We estimate the number of antenna beams per source that are required to reach a specified sensitivity in deep surveys. The results of our calculations are compared with data derived from catalogs and numerous surveys.     

Progress in low-frequency radio astronomy and I.S. Shklovskii’s contribution to its development

Radio astronomy at decameter wavelengths is currently undergoing very active development. Large-scale, new generation low-frequency radio telescopes are being constructed and already used in many countries around the world. As before, the largest, most sensitive, and most versatile telescope at decameter wavelengths is the Ukrainian UTR-2 radio telescope operating at 8–32 MHz, which has an effective area of more than 10⁵ m² and an angular resolution of about 0.5?, as well as the URAN interferometric system based on the UTF-2. Many studies that have been carried out on these facilities have been based on important results and far-sighted predictions of Shklovskii. These include, in particular, studies of dynamical spectra and the brightness distributions of the sporadic and quiescent decameter radio emission of the hot solar corona, complex, broadband radio spectroscopy of the interstellar medium, and multi-frequency monitoring of secular decreases in the flux densities of supernova remnants. The coordinated use of highly effective existing and newly constructed radio telescopes joined into ground networks, as well as specialized space missions, are opening new prospects for low-frequency radio astronomy.     

Monitoring of flux densities of bright radio sources in a narrow strip of sky at 1420 MHz

The flux densities of a uniform sample of extragalactic radio sources in a narrow strip of sky (1° wide in declination with a total area of about 0.1 sr) were monitored for 105 days at 1420 MHz using the 64-m Kalyazin radio telescope. In total, 32 bright sources with flux densities greater than 0.6 Jy have been studied. The statistical properties of the observed flux-density variations are discussed. We detected variability at the 95% confidence level according to the χ ² test in 4C +27.15 (0516+2740). The flux density variations in this source show the pattern typical of extreme scattering events. There are reasons to classify this source as a blazar.     

A RATAN-600 Zenith-Field sky survey. Catalog of radio sources

A catalog of radio sources detected in a deep RATAN-600 survey is presented. The catalog was obtained in the region 0^h ≤ RA_2000.0 ≤ 24^h, Dec_2000.0 = 41°30′42″ ± 2′, at the declination of the bright radio source 3C 84. There were nine sessions of multi-wavelength observations at wavelengths λ = 1–55 cm, and more than 300 daily scans were accumulated at each wavelength. This is the first stage in the reduction of an extensive database accumulated by the Cosmological Gene Project. The RATAN-600 Zenith Field (RZF) catalog was obtained at the central wavelength of 7.6 cm, and contains 437 radio sources, virtually all of which have been identified with NVSS objects. Most of the flux densities for the catalog sources are above the 5σ level. Noise from faint (mainly new) background sources at a level of about 0.8 mJy has been detected. The minimum flux density of the catalog, 2.5 mJy, is comparable to the flux-density linit of the NVSS catalog. The catalog is more than 80% complete for sources with flux densities >3 mJy.     

Objects from a cross-identification of the IRAS and low-frequency Texas radio catalogs

A cross-identification of objects in the low-frequency (365 MHz) Texas radio catalog and in IRAS catalogs at four infrared wavelengths has yielded a list of 715 objects for further studies. Objects with steep spectra for which the difference in the centers of gravity of the radio and infrared sources was less than 3″ were selected from this list. Seventeen of the objects have been observed at six wavelengths using the RATAN-600 radio telescope. Spectra of nine objects from the initial list for which there were candidate optical counterparts were obtained using the 2.1-m telescope of the INAOE. The results of these observations are discussed. The presence of steep spectral indices for the radio sources is confirmed. The possible optical counterparts include interacting galaxies, an infrared galaxy, two emission-line galaxies, and a candidate BL Lac object. Optical images of the optical counterparts are presented together with radio and optical spectra.     

Secular decrease of the radio flux density of Cassiopeia A determined from long-term observations. Observations at 38, 81.5, and 151.5 MHz

Long-term measurements of the radio flux density of the young supernova remnant Cassiopeia A relative to the radio galaxy Cygnus A have been carried out at 38 MHz (1987–2004) and 151.5 MHz (1980–2004). Using other data from the literature, we find a secular decrease of the radio flux density of Cassiopeia A at the rates d(38 MHz) = ?0.79 ± 0.14% yr^?1 (for 1956–2004) and d(151.5 MHz) = ?0.83 ± 0.04% yr^?1 (for 1966–2004). Based on measurements made in 1997 and 1998 and data from the literature, this secular decrease at 81.5 MHz is d(81.5 MHz) = ?0.86 ± 0.14% yr^?1 (for 1966–1998). Absolute flux densities of Cassiopeia A at 38 and 151.5 MHz for epoch 2005.5 are calculated based on the relative flux density of Cassiopeia A and the spectrum of Cyg A, which is approximated using an empirical formula at meter and decameter wavelengths.     

Phenomenological model for the evolution of radio galaxies such as Cygnus A

A phenomenological model for the evolution of classical radio galaxies such as Cygnus A is presented. An activity cycle of the host galaxy in the radio begins with the birth of radio jets, which correspond to shocks on scales ～1 pc (the radio galaxy B0108+388). In the following stage of the evolution, the radio emission comes predominantly from formations on scales of 10–100 pc, whose physical parameters are close to those of the hot spots of Cygnus A (this corresponds to GHz-peaked spectrum radio sources). Further, the hot spots create radio lobes on scales of 10³–10⁴ pc (compact steep-spectrum radio sources). The fully formed radio galaxies have radio jets, hot spots, and giant radio lobes; the direction of the jets can vary in a discrete steps with time, creating new hot spots and inflating the radio lobes (as in Cygnus A). In the final stage of the evolutionary cycle, first the radio jets disappear, then the hot spots, and finally the radio lobes (similar to the giant radio galaxies DA 240 and 3C 236). A large fraction of radio galaxies with repeating activity cycles is observed. The close connection between Cygnus A-type radio galaxies and optical quasars is noted, as well as similarity in the cosmological evolution of powerful radio galaxies and optical quasars.     

Frequency dependence of the evolution of the radio emission of the supernova remnant Cas A

Many-year measurements of the radio flux of the young supernova remnant Cassiopeia A relative to the radio galaxy Cygnus A were continued at 290 and 151.5 MHz. The new data are used together with previously published observations carried out at decameter, meter, centimeter, and millimeter wavelengths to derive the frequency dependence of the secular variation of the radio flux density of Cas A: $d_\nu [\% year^{ - 1} ] = - (0.63 \pm 0.02) + (0.04 \pm 0.01)\ln \nu [GHz] + (1.51 \pm 0.16) \times 10^{ - 5} (\nu [GHz])^{ - 2.1} $ . The observed slowing of the secular variations with decreasing frequency at decameter wavelengths can be explained by a decrease in the optical depth of a remnant HII zone around Cas A with time due to recombination of hydrogen atoms. The new derived frequency dependence for the rate of the secular decrease, absolute and relative measurements of the radio flux density of Cas A carried out over the last 25 years, and the absolute spectrum of Cyg A are used to construct the spectrum of Cas A in the range 5–250 000 MHz predicted for epoch 2015.5.     

Simultaneous dual-frequency observations of giant radio pulses from the millisecond pulsar B1937+21

Simultaneous dual-frequency observations of giant radio pulses from the millisecond pulsar B1937+21 were performed for the first time in January–February 2002 on the Westerbork Synthesis Radio Telescope (2210–2250 MHz) and the 64-m Kalyazin radio telescope (1414–1446 MHz). The total observing time was about three hours. Ten giant pulses with peak flux densities from 600 to 1800 Jy were detected at 2210–2250 MHz, and fifteen giant pulses with peak flux densities from 3000 to 10000 Jy were observed at 1414–1446 MHz. No events were found to occur simultaneously at both frequencies. Thus, the observed radio spectra of individual giant pulses of this pulsar are limited in frequency to scales of about \(\frac{{\Delta v}}{v} < 0.5\). The duration of the giant pulses is less than 100 ns and is consistent with the expected scattering timescale in these frequency ranges. Instantaneous radio spectra of the detected giant pulses were compared with the diffractive spectra obtained from ordinary pulses of the pulsar. In some cases, considerable deviations of the radio spectra of the giant pulses from the diffractive spectrum were revealed, which can be interpreted as indicating temporal structure of the giant pulses on timescales of 10–100 ns.     

Spectrum of the variable component of the radio source J0157+7442

The spectrum of the maximum values of the variable component of the radio source J0157+7442 is presented. The flux densities obtained on the RATAN-600 radio telescope at centimeter wavelengths in 2009 and 2010 are minimum for all observations of the source, and it was assumed that the variable component was absent in these years. After subtracting these RATAN-600 data from the upper envelope of all available flux density data, the spectrum of the variable component was obtained. The form of the spectrum of the variable component is typical of a nonuniform, spherically symmetric source with synchrotron self-absorption.     

Parameters of the turbulence of the interplanetary plasma derived from scintillation observations of the quasar 3C 48 at the solar-activity minimum

Temporal spectra of interplanetary scintillations of the strong radio source 3C 48 based on 111 MHz observations on the Large Scanning Antenna of the Lebedev Physical Institute obtained near the solar-activity minimum are analyzed. Measurements of the temporal spectrum of the scintillations are used to estimate the angular size of the source, the velocity of inhomogeneities, and the power-law index for the spatial spectrum of the turbulence in the interplanetary plasma. The mean angular size of the source is θ ₀ = 0.326″ ± 0.016″, and the mean index for the three-dimensional turbulence spectrum is n = 3.7 ± 0.2. There is some evidence that n decreases in the transition from the fast, high-speed to the slow, low-latitude solar wind.     

Radio emission of RRAT pulsars at 111 MHz

Observations of the RRAT pulsars J0627+16, J0628+09, J1819?1458, J1826?1419, J1839?01, J1840?1419, J1846?0257, J1848?12, J1850+15, J1854+0306, J1919+06, J1913+1330, J1919+17, J1946+24, and J2033+00 observed earlier on the 64-m Parkes telescope (Australia) and the 300-m Arecibo radio telescope (Puerto Rico) at 1400 MHz were conducted at 111 MHz on the LSA radio telescope of the Pushchino Radio Astronomy observatory in 2010–2012. A characteristic feature of these pulsars is their sporadic radio emission during rare active epochs and the absence of radio emission during long time intervals. No appreciable flare activity of these pulsars was detected in the Pushchino observations. However, processing the observations using the Fast Folding Algorithm taking into account known information about the pulsar dispersion measures and periods shows that, even during quiescent intervals, the majority of the studied pulsars generate weak radio pulses with a period corresponding to that of the radio emission of the sporadic pulses observed at active epochs. The flux of this radio emission does not exceed 100 mJy at the pulse peak, even at the low frequency of 111 MHz. This considerably hinders detection of the radio emission of RRAT pulsars at high frequencies, since the radio fluxes of RRAT pulsars decreases with increasing frequency.     

Spectra of Standard Radio Sources at the Current Epoch

The results of observations aimed at determining the spectra of standard radio sources in an “artificial-Moon” absolute flux scale at epoch 2015.5 at wavelengths of 3.5–18 cm are presented. The sample includes sources with small angular sizes, used as flux-density standards for the calibration of various types of antennas, including large synthesized apertures, over a wide range of frequencies. The artificial-Moon flux scale is constructed in a way that makes it the most accurate of existing scales, and the only scale for which source spectra are not deformed. The methods used to construct the articial-Moon flux scale are briefly described. Repeat measurements of the spectra of standard sources, required to preserve the accuracy of the flux scale, were carried out in 2014–2017 (at a mean epoch of 2015.5) on the 32-m telescope of the Svetloe Observatory of the Institute of Applied Astronomy at four wavelengths: 18, 13, 6.2, and 3.5 cm. The characteristic properties of the measurement methods are discussed, and the data obtained are compared with data from earlier epochs.     

The microwave radiation of the corona above a large single sunspot in right- and left-circular polarization

Results of a study of the corona above a large sunspot in the active region NOAA 10105 with a penumbra size of ～70″ observed in September 2002 are reported. Maps of the active region and emission spectra were constructed using observational data from the NoRH, SSRT, and RATAN-600 telescopes. The sizes and brightness temperatures of the microwave emission above the sunspot are determined. SOHO/MDI and Kitt Peak magnetograms, as well as CaII K line images obtained at the Meudon Observatory, are compared. The derived characteristics are interpreted as cyclotron emission of thermal plasma, assuming a dipole structure for themagnetic field. A stable darkening at the sunspot center observed at short wavelengths and only in the ordinary emission mode was detected. A jump-like change was observed in the structure of the sunspot source in the ordinary emission mode, due to an increase in the size and spectral flux density. These results demand a fundamental correction of model concepts about cyclotron emission sources above sunspots, since they are at variance with the initial assumptions. It is suggested that, at the top of the transition region, the cyclotron emission source may be represented only by the third gyrolevel, but is observed in the extraordinary and ordinary emission modes (in contrast to the generally accepted model, which has a combination of the second and third gyrolevels). Taking into account the new observational data may allow us to refine model distributions of the main parameters of the coronal plasma above sunspots (the electron temperature and density) and information about the character of the magnetic field.     

Correlations between the development of a flare in the Blazar 3C 454.3 in the radio and optical

Radio and optical data are used to analyze the development of the flare in the blazar 3C 454.3 observed in 2004–2007. A detailed correspondance between the optical and radio flares is established, with a time delay that depends on the observing frequency. The variation of the delay of the radio flare relative to the optical flare is opposite to the dispersion delay expected for the propagation of radiation in the interstellar medium, testifying to an intrinsic origin for the observed outburst. Small-scale flux variations on time intervals of 5–10 days in the millimeter and optical are also correlated, with a time delay of about ten months. This may provide evidence for a single source generating the radiation at all wavelengths. Rapid flux fluctuations in the radio and optical that are correlated with the indicated time delays could be associated with inhomogeneities in the accretion disk. Detailed studies of the flux variations of Active Galactic Nuclei (AGN) can be used to analyze the structure of the accretion disk. A model for the energy release in AGN that is not associated purely with accretion onto supermassive black holes is proposed. As is the case for other active members of the AGN family, estimates of the lifetime of the binary black-hole system in 3C 454.3 suggest that this object is in a stage of its evolution that is fairly close to the coalescence of its black holes. The energy that is released as the companion of the central black hole loses orbital angular momentum is sufficient to explain the observed AGN phenomena. The source of primary energy release could be heating of the gas behind shock fronts that arise due to the friction between the companion black hole and the ambient gaseous medium. The orbit of the companion could be located at the periphery of the accretion disk of the central body at its apocenter and plunge more deeply into the accretion disk at its pericenter, inducing flares at all wavelengths. Energy-release parameters such as the temperature and density of the heated gas are estimated for 3C 454.3. The model considered assumes omnidirectional radiation of the medium in the presence of a magnetic field. The radiation corresponding to the minimum flux level (base level) could represent omnidirectional radiation due to the orbit of the moving companion. The fraction of the energy that is transferred to directed jets is small, comprising 1–2% of the total energy released due to the loss of orbital angular momentum by the companion.     

Physical conditions in steep-spectrum radio sources

Interplanetary scintillation observations of the radio sources 4C 31.04, 3C 67, 4C 34.07, 4C 34.09, OE 131, 3C 93.1, OF 247, 3C 147, 3C 173, OI 407, 4C 68.08, 3C186, 3C 190, 3C 191, 3C 213.1, 3C 216, 3C 237, 3C 241, 4C 14.41, 3C 258, and 3C 266 have been carried out at 102 MHz. Scintillations were detected for nearly all the sources. The integrated flux densities and flux densities of the scintillating components are estimated. Nine of the 21 sources have a low-frequency turnover in their spectra; three of the sources have high-frequency turnovers. The physical parameters are estimated for sources with turnovers in the spectra of their compact components. In most of the quasars, the relativistic-plasma energy exceeds the magnetic-field energy, while the opposite is true of most of the radio galaxies. Empirical relations between the size of the compact radio source and its magnetic field and relativistic-electron density are derived.