The asymmetry coefficient for interstellar scintillation of extragalactic radio sources

Comparing the asymmetry coefficients γ and scintillation indices m for observed time variations of the intensity of the radiation of extragalactic sources and the predictions of theoretical models is a good test of the nature of the observed variations. Such comparisons can be used to determine whether flux density variations are due to scintillation in the interstellar medium or are intrinsic to the source. In the former case, they can be used to estimate the fraction of the total flux contributed by the compact component (core) whose flux density variations are caused by inhomogeneities in the interstellar plasma. Results for the radio sources PKS 0405-385, B0917+624, PKS 1257-336, and J1819+3845 demonstrate that the scintillating component in these objects makes up from 50 to 100% of the total flux, and that the intrinsic angular sizes of the sources at 5 GHz are 10–40 microarcseconds. The characteristics of the medium giving rise to the scintillations are presented.     

Studies of the Angular Structure of Scintillating Radio Sources at 111 MHz

The angular sizes and compactnesses of 53 scintillating radio sources observed at 111 MHz on the Large Phased Array of the Lebedev Physical Institute are estimated. The parameters of the angular structures of the sources are estimated using a new method based on a joint analysis of the scintillation index and the asymmetry coefficient for the statistical distribution of the intensity fluctuations. The asymmetry coefficient for scintillations of a point source is estimated based on an analysis of observational data for turbulence in the solar wind. Different methods for estimating source angular sizes based on observations of interplanetary scintillations are compared. It is shown that the proposed new method is suitable for sources with angular sizes up to 1″. The accuracy of the estimated angular sizes and compactnesses of the sources is about 40%.

     

Interstellar scintillation of the radio source associated with the gamma-ray burst of May 8, 1997

The variability of the radio source associated with the gamma-ray burst of May 8, 1997, detected using the VLA, is analyzed. This variability can be explained as weak scintillations at 4.86 and 8.46 GHz and the refractive component of saturated scintillations at 1.43 GHz. Possible distances for the source are discussed. The scintillation parameters are in best agreement with the observations if the source is at a cosmological distance and has an angular size ～2 microarcseconds (µas) at 4.86 GHz and an expansion speed of the order of 25 µas/year.     

Asymmetry function of interstellar scintillations of pulsars

A new method for separating intensity variations of a source’s radio emission having various physical natures is proposed. The method is based on the joint analysis of the structure function of intensity variations and the asymmetry function, which is a generalization of the asymmetry coefficient and which characterizes the asymmetry of the distribution function of intensity fluctuations on various scales for inhomogeneitiesin the diffractive scintillation pattern. Relationships for the asymmetry function in the cases of a logarithmic normal distribution of the intensity fluctuations and a normal distribution of the field fluctuations are derived. Theoretical relationships and observational data on interstellar scintillations of pulsars (refractive, diffractive, and weak scintillations) are compared. The data for PSR B0329+54, B1133+16, B1642-03, and B1933+16 pulsars were used for comparison. Pulsar scintillations match the behavior expected for a normal distribution of field fluctuations (diffractive scintillation) or logarithmic normal distribution of intensity fluctuations (refractive and weak scintillation). Analysis of the asymmetry function is a good test for distinguishing scintillations against the background of variations that have different origins.     

The turbulence spectrum of the interstellar plasma toward the pulsars PSR B0809+74 and B0950+08

The interstellar scintillation of the pulsars PSR B0809+74 and B0950+08 have been studied using observations at low radio frequencies (41, 62, 89, and 112 MHz), and the characteristic temporal and frequency scales for diffractive scintillations at these frequencies determined. A comprehensive analysis of the frequency and temporal structure functions reduced to a single frequency shows that the spectra of the inhomogeneities of the interstellar plasma toward both pulsars are described by a power law. The index of the interstellar plasma fluctuation spectrum toward PSR B0950+08 (n = 3.00 ± 0.05) differs appreciably from the Kolmogorov index. The spectrum toward PSR B0809+74 is a power law with index n = 3.7 ± 0.1. Strong angular refraction has been detected toward PSR B0950+08. Analysis of the distribution of inhomogeneities along the line of sight indicates that the scintillations of PSR B0950+08 take place in a turbulent layer with an enhanced electron density localized approximately 10 pc from the observer. The distribution of inhomogeneities for PSR B0809+74 is quasi-uniform. The mean square fluctuations of the electron density are estimated for inhomogeneities with characteristic scale ρ ₀ = 10⁷ m along the directions toward four pulsars. The local turbulence in the 10-pc layer is a factor of 20 higher on this scale than in the extended region responsible for the scintillations of PSR B0809+74.     

Study of Bright Compact Radio Sources of the Northern Hemisphere at 111 MHz

The search for compact components of strong ($${{S}_{{{\text{int}}}}} \geqslant 5$$ Jy at 102.5 MHz) discrete radio sources from the Pushchino catalogue was carried out using the method of interplanetary scintillation. A total of 3620 sources were examined, and 812 of them were found to harbor compact (scintillating) components. Estimates of fluctuations of the flux density of these compact components were derived from the scintillation index ($${{m}_{{\max}}}$$) corresponding to an elongation of 25°. The angular size and compactness of 178 sources with compact components were estimated. Scintillation indices of sources corresponding to the compact component ($${{m}_{0}}$$) and flux densities of compact components were determined. It was demonstrated that slow variations of the spatial distribution of interplanetary plasma, which are related to the 11-year cycle of solar activity, may exert a systematic influence on the estimates of angular sizes of sources. Coefficients compensating the deviation from the spherical symmetry of solar wind in the estimates of angular sizes were found using the coefficient of asymmetry of the statistical distribution of intensity fluctuations. The study of correlations between the parameters of sources in the sample revealed that the maximum value of the scintillation index decreases as the integrated flux increases, while the angular size has no marked dependence on the integrated flux.     

Monitoring of the turbulent solar wind with the upgraded Large Phased Array of the Lebedev Institute of Physics: First results

The design properties and technical characteristics of the upgraded Large Phased Array (LPA) are briefly described. The results of an annual cycle of observations of interplanetary scintillations of radio sources on the LPA with the new 96-beam BEAM 3 system are presented. Within a day, about 5000 radio sources displaying second-timescale fluctuations in their flux densities due to interplanetary scintillations were observed. At present, the parameters of many of these radio sources are unknown. Therefore, the number of sources with root-mean-square flux-density fluctuations greater than 0.2 Jy in a 3° × 3° area of sky was used to characterize the scintillation level. The observational data obtained during the period of the maximum of solar cycle 24 can be interpreted using a three-component model for the spatial structure of the solar wind, consisting of a stable global component, propagating disturbances, and corotating structures. The global component corresponds to the spherically symmetric structure of the distribution of the turbulent interplanetary plasma. Disturbances propagating from the Sun are observed against the background of the global structure. Propagating disturbances recorded at heliocentric distances of 0.4–1 AU and at all heliolatitudes reach the Earth’s orbit one to two days after the scintillation enhancement. Enhancements of ionospheric scintillations are observed during night-time. Corotating disturbances have a recurrence period of 27^d. Disturbances of the ionosphere are observed as the coronal base of a corotating structure approaches the western edge of the solar limb.     

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.     

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.     

Some results of night-time scintillations at low latitude

Ionospheric scintillation observations of VHF radio signals from FLEETSAT satellite (73°E longitude) at Bhopal from January 1990 to December 1990 have been used to study the characteristic variations of scintillation activity. It is found that scintillation occurrence is essentially a night-time phenomenon and day-time scintillations are very rare. Annual average nocturnal variation of percentage occurrence of scintillations shows maximum at around 2100–2200 hours LT. Seasonally, scintillations are most prominent during equinoxes and least during summer. Geomagnetic disturbances tend to decrease the occurrence of scintillations in the pre-midnight period.     

Study of compact radio sources using interplanetary scintillations at 111 MHz. The Pearson-Readhead sample

A complete sample of radio sources has been studied using the interplanetary scintillation method. In total, 32 sources were observed, with scintillations detected in 12 of them. The remaining sources have upper limits for the flux densities of their compact components. Integrated flux densities are estimated for 18 sources.     

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.     

The detection of coronal mass ejections in the interplanetary medium using scintillation observations

Daily observations of scintillating radio sources obtained from July 2011 through June 2012 on the Big Scanning Antenna of the P.N. Lebedev Physical Institute at 111 MHz using a 16 beams system are analyzed. Variations in the observed scintillation indices are compared with data on solar X-ray flares and geomagnetic disturbances. Comparison of the observed scintillation indices on successive days enables the detection of most propagating disturbances associated with coronal events of class M5.0 and higher.     

Corotating structures in the solar wind from 111-MHz observations of interplanetary scintillations at large elongations

Results of continuous 111 MHz observations of interplanetary scintillations of the strong radio source 3C 48 at elongations larger than 80? out on the Large Phased Array (LPA) of the Lebedev Physical Institute are reported. The data were taken during a four-year interval, from 2012 to 2015, near the maximum of the 24th solar-activity cycle. The averaged elongation dependence of the scintillation index and similar dependences for individual years during the approach and recession phases suggest the presence of a periodic modulation with a 26-day period, which is masked by day-to-day variations. This periodic modulation can be explained by the existence of a long-lived region of enhanced plasma density adjacent to the solar equator during the solar-activity maximum. It is shown that the scintillation timescale increases in the transition to elongations exceeding 90?.     

Extended component in the quasar 3C 380

Results of radio interferometric observations of the quasar 3C 380 carried out on the URAN interferometers at decameter wavelengths and on the aperture synthesis radio telescope VLA at meter wavelengths are reported. The spectral index of an extended lobe about 10″ in size is considerably lower than at decimeter wavelengths. Below ～ 100 MHz, the ratio of the emission from the compact components associated with hot spots in the radio lobe to the total flux of the source decreases due to synchrotron self-absorption at hot spots, whose flux density at 20 MHz does not exceed 65 Jy. A halo with a full width at a half-maximum of about 40″ was detected, whose angular extent considerably exceeds the total source size measured at shorter wavelengths.     

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.     

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.     

Radio sounding of the circumsolar plasma using polarized pulsar pulses

We present the results of radio sounding observations probing the inner solar wind near the minimum of the solar-activity cycle, using polarized pulses from PSR B0525+21 and PSR B0531+21 received when the lines of sight toward these pulsars were close to the Sun. The observations were obtained in June 2005 and June 2007 on the Large Phased Array of the Lebedev Physical Institute at 111 MHz. An upper limit for the scattering of giant pulses from PSR B0531+21 due to their passage through the turbulent solar-wind plasma is determined. The arrival-time delays for pulses from PSR B0531+21 are used to derive the radial dependence of the mean density of the circumsolar plasma. The resulting density distribution indicates that the acceleration of fast, high-latitude solar-wind outflows continues to heliocentric distances of 5–10R _⊙, where R _⊙ is the solar radius. The mean plasma density at heliocentric distances of about 5R _⊙ is 1.4 × 10⁴ cm^?3, substantially lower than at the solar-activity maximum. This is associated with the presence of polar coronal holes. The Faraday rotation measure at heliocentric distances of 6–7R _⊙ is estimated. Deviations of the spatial distribution of the magnetic field from spherical symmetry are comparatively modest in the studied range of heliocentric distances.     

Global Structure of the Solar Wind in a of Decreasing Solar Activity from Interplanetary-Scintillation Monitoring Data

An analysis of data from three years of monitoring of interplanetary scintillations in 2015–2017 during a phase of decreasing solar activity is presented. The observations were carried out on the Large Scanning Antenna of the Lebedev Physical Institute at 111 MHz. During the period considered, the spatial distriution of the scintillation level was close to spherically symmetrical, on average, and did not undergo any strong time variations on scales of months or years. The monthly-mean scintillation level is not correlated with theWolf number.

     

Turbulent interstellar plasma and ultrahigh angular resolution in radio astronomy

The effects of the modulation of radio waves during their passage through turbulent interstellar plasma on measurements of the structure of radio sources made with ultrahigh angular resolution using space radio interferometers are considered. Typical scattering angles?_{scat, pl} for an extragalactic radio source at various wavelengths are estimated from scattering observations for pulsars and extragalactic sources. The (?₀λ) plane, where ?₀ is the source size and λ is the wavelength of the radio emission, can be divided into five regions, in which different regimes of radio-wave modulation and image reconstruction are realized. Possibilities for image reconstruction in each of these regions are investigated.