Giant Ukrainian Radio Telescope Research Articles

Free–free absorption parameters of Cassiopeia A from low-frequency interferometric observations

Context. Cassiopeia A is one of the most extensively studied supernova remnants (SNRs) in our Galaxy. The analysis of its continuum spectrum through low-frequency observations plays an important role for understanding the evolution of the radio source and the propagation of synchrotron emission to observers through the SNR environment and the ionized interstellar medium. Aims. In this paper we present measurements of the integrated spectrum of Cas A to characterize the properties of free–free absorption toward this SNR. We also add new measurements to track its slowly evolving and decreasing integrated flux density. Methods. We used the Giant Ukrainian Radio Telescope (GURT) to measure the continuum spectrum of Cassiopeia A within the frequency range of 16–72 MHz. The radio flux density of Cassiopeia A relative to the reference source of the radio galaxy Cygnus A was measured in May–October 2019 with two subarrays of the GURT, used as a two-element correlation interferometer. Results. We determine magnitudes of emission measure, electron temperature, and an average number of charges of the ions for both internal and external absorbing ionized gas toward Cassiopeia A. Generally, their values are comparable, albeit with slight differences, to recently presented values. In the absence of clumping, we find that the unshocked ejecta of M = 2.61 M⊙ at the electron density of 15.3 cm−3 has a gas temperature of T ≈ 100 K. If the clumping factor is 0.67, then the unshocked ejecta of 0.96 M⊙ has an electron density of 18.7 cm−3. Conclusions. The integrated flux density spectrum of Cassiopeia A obtained with the GURT interferometric observations is consistent with the theoretical model within measurement errors and also reasonably consistent with other recent results in the literature.

Parker Solar Probe detects solar radio bursts related with a behind–the–limb active region

Context.The interpretation of solar radio bursts observed by Parker Solar Probe (PSP) in the encounter phase plays a key role in understanding intrinsic properties of the emission mechanism in the solar corona. Lower time–frequency resolution of the PSP receiver can be overcome by simultaneous ground–based observations using more advanced antennas and receivers.Aims.In this paper we present such observations for which the active active region 12 765, begetter of type III, J, and U solar bursts, was within sight of ground–based instruments and behind the solar limb of the PSP spacecraft.Methods.We used a subarray of the Giant Ukrainian Radio Telescope to get the spectral properties of radio bursts at the frequency range of 8–80 MHz, as well as the PSP radio instruments with a bandwidth of 10.5 kHz–19.2 MHz, during solar observations on June 5, 2020.Results.We directly detected the radio events initiated by the active region behind the solar limb of the PSP spacecraft, using special conditions in the solar corona, due to the absence of active regions from the PSP side. Following the generation mechanism of solar radio emission, we refined the density model for the solar corona above the active region 12765 responsible for the radio bursts. Based on the PSP spacecraft position near the Sun and delays of radio waves between space– and ground–based records, we found the corresponding radio responses on the PSP spectrogram.Conclusions.The absence of sunspots from the PSP side contributes to the propagation of radio waves from a dense loop of the Sun to quiet regions with low densities, through which PSP instruments can detect the radiation.

Properties of Type III and Type IIIb Bursts in the Frequency Band of 8 – 80 MHz During PSP Perihelion at the Beginning of April 2019

Properties of type III and type IIIb bursts in the frequency band of 8-80 MHz observed by the radio telescopes Ukrainian Radio Interferometer of NASU-2 (URAN-2) (Poltava) and Giant Ukrainian Radio Telescope (GURT) (Kharkiv) during the Parker Solar Probe (PSP) perihelion in April 2019 are discussed. These correspond of those that were observed by PSP at frequencies <19 MHz. We analyze dependences of drift rates and durations on frequency for these bursts. We show that drift rate dependences on frequency agree well with those derived from the Newkirk corona if source velocities are between 0.17 and 0.2 c for both type IIIb bursts and type III bursts under the assumption that the first ones are fundamentals and the second ones are their harmonics. However, all observational dependences are flatter in comparison with the dependences for a Newkirk corona. We assume that this can be related with coronal temperature decreasing at heliocentric distances from 1.35 to 6.5 solar radii. Duration dependencies of type IIIb and type III bursts on frequency in the range of 10-70 MHz are also obtained. We note that the durations and drift rates of type III bursts as well as their dependences on frequency differ essentially from those for type IIIb bursts.

Solar Type U Burst Associated with a High Coronal Loop

An inverted U burst with equally developed ascending and descending branches observed by Giant Ukrainian Radio Telescope (GURT) on 18 April 2017 in meter wavelengths band is discussed. This U burst was attributed to the high coronal loop above the limb active region NOAA 12651. Under the assumption that, associated with the burst, the coronal loop confines isothermal plasma stratified according to a Boltzmann density relation, the geometrical and physical parameters of the loop were estimated. According to our model coronal loops may contain plasma which is up to 20 times denser than the surrounding coronal plasma. In general the proposed model gives the relation between the plasma temperature and the height of the loop in such a way that under the given parameters of the associated U burst, higher loops contain cooler plasma and vice versa. An alternative method of coronal loop height determination was suggested. Assuming that the observed U burst and the preceding type III burst were generated by the same exciter we define the height of the loop from the delay of the former with respect to the latter at certain frequency. We show that defining of coronal loops heights by another independent method, e.g. interferometric or tied-array imaging may solve the uncertainty of the inside-the-loop plasma temperature determination.

An Active Antenna Subarray for the Low-Frequency Radio Telescope GURT—Part I: Design and Theoretical Model

The new Giant Ukrainian Radio Telescope (GURT) intended for operation in 8–80 MHz range is now under construction. It employs an active phased array antenna composed of many dipole subarrays, several of which have been implemented and now are used for radio astronomy observations. In this two-part paper, the electrical and noise parameters of the subarray are studied by numerical and experimental ways. In this part, a theoretical model of the subarray based on the wave theory of noisy multiport networks that takes into account the mutual coupling in the dipole array placed over imperfect ground and the correlation between all existing noise sources is proposed. It is assumed that the initial parameters for this model should be obtained using the correct electromagnetic and circuit simulation. A technique for using of this model to calculate the effective area, radiation efficiency, internal and external noise temperatures, as well as sensitivity of the subarray in terms of system equivalent flux density (SEFD) for all possible beam directions in the frequency band of the GURT radio telescope is described.

An Active Antenna Subarray for the Low-Frequency Radio Telescope GURT—Part II: Numerical Analysis and Experiment

This second part of two articles’ sequence presents the detailed numerical analysis of electrical and noise parameters of a subarray of the Giant Ukrainian Radio Telescope (GURT) designed for operation in the frequency range of 8–80 MHz. The theoretical model of the subarray and calculation technique of its parameters developed in the first part is used. Frequency behaviors of the absorption area, radiation efficiency, and system noise temperature budget of the subarray are studied. Special attention is paid to the subarray sensitivity given in terms of sky noise dominance (SND) and system-equivalent flux density (SEFD). The calculation results show that strong mutual coupling between subarray elements enables a significant increase in the subarray SND and enhancing overall its performance. The frequency dependences of the system noise temperature for 23 beams of the subarray, obtained by in situ measurements, are presented. A good agreement between the measured and calculated results well enough validates the subarray theoretical model.

Solar bursts as can be observed from the lunar farside with a single antenna at very low frequencies

Earth-based observations are complicated by the opacity of Earth's ionosphere at very low frequencies and strong man-made radio frequency interference. This explains long standing interest in building a low frequency radio telescope on the farside of the Moon. Experience from ground-based observations near the ionospheric cutoff in dealing with the interference, ionosphere, and wide-field imaging/dynamic range problems provides crucial information for future radioastronomic experiments on the Moon. In this purpose we observed non-intensive solar bursts on the example of solar drift pairs (DP) at decameter-meter wavelengths with large and small arrays as well as by a single crossed active dipole. We used the large Ukrainian radio telescope UTR-2, the URAN-2 array, a subarray of the Giant Ukrainian radio telescope (GURT) and a single crossed active dipole to get the spectral properties of radio bursts at the frequency range of 8-80 MHz during solar observations on July 12, 2017. Statistical analysis of upper and lower frequencies, at which DPs are recorded, shows that the occurrence of forward DPs is more preferable at lower frequencies of the decameter range of observations in comparison with reverse DPs generated more likely at meter wavelengths. We conclude that DPs can be detected not only by antenna arrays, but even by a single crossed active dipole. Thus the latter antenna has a good potential for future low-frequency radio telescopes on the Moon.

Sensitivity of an Active Antenna Array Element for the Low-Frequency Radio Telescope GURT

The recently developed new generation, low-frequency Giant Ukrainian Radio Telescope (GURT) is built nearby the well-known Ukrainian T-shaped Radio Telescope. The new facility employs a phased antenna array composed of many subarrays of $5 \times 5$ active antenna elements. In this paper, the parameters of the active antenna used as array element are studied, with special attention paid to sensitivity. The electrical and noise parameters are calculated using computer simulation and wave techniques for noise modeling of two-port networks. The results of numerical calculations of the sensitivity are given in terms of the sky noise dominance (SND) and system equivalent flux density of the GURT element within 10–80 MHz. The calculated results are compared with in situ measurements.

The modern radio astronomy network in Ukraine: UTR-2, URAN and GURT

The current status of the large decameter radio telescope UTR-2 (Ukrainian T-shaped Radio telescope) together with its VLBI system called URAN is described in detail. By modernization of these instruments through implementation of novel versatile analog and digital devices as well as new observation techniques, the observational capabilities of UTR-2 have been substantially enhanced. The total effective area of UTR-2 and URAN arrays reaches 200 000 m2, with 24 MHz observational bandwidth (within the 8–32 MHz frequency range), spectral and temporal resolutions down to 4 kHz and 0.5 msec in dynamic spectrum mode or virtually unlimited in waveform mode. Depending on the spectral and temporal resolutions and confusion effects, the sensitivity of UTR-2 varies from a few Jy to a few mJy, and the angular resolution ranges from ~ 30 arcminutes (with a single antenna array) to a few arcseconds (in VLBI mode). In the framework of national and international research projects conducted in recent years, many new results on Solar system objects, the Galaxy and Metagalaxy have been obtained. In order to extend the observation frequency range to 8–80 MHz and enlarge the dimensions of the UTR-2 array, a new instrument – GURT (Giant Ukrainian Radio Telescope) – is now under construction. The radio telescope systems described herein can be used in synergy with other existing low-frequency arrays such as LOFAR, LWA, NenuFAR, as well as provide ground-based support for space-based instruments.

Decameter U-burst Harmonic Pair from a High Loop

The results of the first observations of solar sporadic radio emission within 10 – 70 MHz by the Giant Ukrainian Radio Telescope (GURT) are presented and discussed. Observations in such a wide range of frequencies considerably facilitate the registration of harmonic pairs. The solar U-burst harmonic pair observed on 8 August 2012 is analyzed. The burst key features were determined. Among them, the time delay between the fundamental and harmonic emissions was of special interest. The fundamental emission was delayed for 7 s with respect to the harmonic emission. A model for explaining the occurrence of such a delay is proposed, in which the emission source is located inside a magnetic loop containing plasma of increased density. In this case, the delay appears due to the difference in group velocities of electromagnetic waves at the fundamental and the harmonic frequencies.