Low Radio Frequencies Research Articles

Features of electromagnetic and noise environment in the workplace of the staff of MRI departments: problems of hygienic regulation and control

Introduction. Magnetic resonance imaging (MRI) is one of the most dynamically developing, high-tech diagnostic methods. At the same time, the specific features of the electromagnetic environment and noise exposures at the medical and technical personnel operating and maintaining MRI workplaces are not well understood and are currently not taken into account when conducting a hygienic assessment of working condition in the Russian Federation. The aim of study is scientific substantiation of the need to improve the hygienic assessment of electromagnetic and noise conditions in the workplace of the staff of MRI departments. Materials and methods. An analytical method to evaluate the current domestic and foreign regulatory and methodological documents that determine the hygienic assessment requirements for electromagnetic environment and noise in MRI departments was used. More than 4 thousand measurements of the intensity of electromagnetic fields and noise in 88 rooms in Moscow equipped with different types of MRI (from 0,1 mT to 3 T) and studies of the performing duration various types of work and electromagnetic field and noise exposure on personnel was done. The amplitude-time and frequency parameters of the variables of the electromagnetic field (EMF) and sound pressure detailed studies have been carried out. Results. Significant differences in controlled indicators, acceptable exposure levels, and control methods has revealed by critical analysis of domestic and foreign hygienic regulatory and methodological documents. The studies showed that the medical and technical personnel of MRI departments are exposed with high spatial and temporal gradients static magnetic fields and extreme levels of noise during diagnostic work in the immediate vicinity of the tomograph magnet. The presence of time-varying magnetic fields in a wide range of low frequencies and radio frequencies having a complicated (pulsed) waveform is a feature of the electromagnetic environment in MRI. New standardized control parameters and limit levels for low-frequency ranges magnetic fields for their adequate hygienic assessment are proposed. Conclusion. The studies made it possible to substantiate the proposals the improving of electromagnetic field and noise hygienic assessment at the MRI staff workplaces.

Percolation and nanosecond fluctuators in V2O3 films within the metal–insulator transition

Vanadium sesquioxide (V2O3) exhibits a metal–insulator transition (MIT) at 160 K between a low temperature, monoclinic, antiferromagnetic Mott insulator and a high temperature, rhombohedral, paramagnetic, metallic phase. In thin films, a percolative transition takes place over a finite temperature range of phase coexistence. We study the fluctuating dynamics of this percolative MIT by measuring voltage noise spectra at both low frequencies (up to 100 kHz) and radio frequencies (between 10 MHz and 1 GHz). Noise intensity quadratic in bias is observed in the MIT region, as expected for resistive fluctuations probed nonperturbatively by the current. The low frequency noise resembles flicker-type 1/fβ noise, often taking on the form of Lorentzian noise dominated by a small number of fluctuators as the volume fraction of the insulating phase dominates. Radio frequency noise intensity also quadratic in the bias current allows the identification of resistance fluctuations with lifetimes below 1 ns, approaching timescales seen in non-equilibrium pump–probe studies of the transition. We find quantitative consistency with a model for fluctuations in the percolative fraction. The thermodynamics of the MIT suggests that dominant fluctuations are ones that alter small volumes affecting the connectivity of domain boundaries. This noise serves as a sensitive and nonperturbative probe for the dynamics of switching phenomena in this system.

The All-Sky SignAl Short-Spacing INterferometer (ASSASSIN) – I. Global-sky measurements with the Engineering Development Array-2

ABSTRACT Aiming to fill a crucial gap in our observational knowledge of the early Universe, experiments around the world continue to attempt to verify the claimed detection of the redshifted 21-cm signal from Cosmic Dawn by the EDGES experiment. This sky-averaged or ‘global’ signal from neutral hydrogen should be detectable at low radio frequencies (50–200 MHz), but is difficult to measure due to bright foreground emission and difficulties in reaching the required levels of instrumental-calibration precision. In this paper, we outline our progress toward using a novel new method to measure the global redshifted 21-cm signal. Motivated by the need to use alternative methods with very different systematic errors to EDGES for an independent result, we employ an array of closely spaced antennas to measure the global-sky signal interferometrically, rather than using the conventional approach with a single antenna. We use simulations to demonstrate our newly developed methods and show that, for an idealized instrument, a 21-cm signal could theoretically be extracted from the visibilities of an array of closely spaced dipoles. We verify that our signal-extraction methods work on real data using observations made with a Square-Kilometre-Array-like prototype, the Engineering Development Array-2. Finally, we use the lessons learned in both our simulations and observations to lay out a clear plan for future work, which will ultimately lead to a new global redshifted 21-cm instrument: the All-Sky SignAl Short-Spacing INterferometer (ASSASSIN).

Tight constraints on the excess radio background at z = 9.1 from LOFAR

ABSTRACT The ARCADE2 and LWA1 experiments have claimed an excess over the cosmic microwave background (CMB) at low radio frequencies. If the cosmological high-redshift contribution to this radio background is between 0.1 per cent and 22 per cent of the CMB at 1.42 GHz, it could explain the tentative EDGES low-band detection of the anomalously deep absorption in the 21-cm signal of neutral hydrogen. We use the upper limit on the 21-cm signal from the Epoch of Reionization (z = 9.1) based on 141 h of observations with LOFAR to evaluate the contribution of the high-redshift Universe to the detected radio background. Marginalizing over astrophysical properties of star-forming haloes, we find (at 95 per cent CL) that the cosmological radio background can be at most 9.6 per cent of the CMB at 1.42 GHz. This limit rules out strong contribution of the high-redshift Universe to the ARCADE2 and LWA1 measurements. Even though LOFAR places limit on the extra radio background, excess of 0.1–9.6 per cent over the CMB (at 1.42 GHz) is still allowed and could explain the EDGES low-band detection. We also constrain the thermal and ionization state of the gas at z = 9.1, and put limits on the properties of the first star-forming objects. We find that, in agreement with the limits from EDGES high-band data, LOFAR data constrain scenarios with inefficient X-ray sources, and cases where the Universe was ionized by stars in massive haloes only.

Tools21cm: A python package to analyse the large-scale 21-cm signal from the Epoch of Reionization and Cosmic Dawn

Giri et al., (2020). Tools21cm: A python package to analyse the large-scale 21-cm signal from the Epoch of Reionization and Cosmic Dawn. Journal of Open Source Software, 5(52), 2363, https://doi.org/10.21105/joss.02363

Global 21 cm Signal Extraction from Foreground and Instrumental Effects. II. Efficient and Self-consistent Technique for Constraining Nonlinear Signal Models

Abstract We present the completion of a data analysis pipeline that self-consistently separates global 21 cm signals from large systematics using a pattern recognition technique. This pipeline will be used for both ground and space-based hydrogen cosmology instruments. In the first paper of this series, we obtain optimal basis vectors from signal and foreground training sets to linearly fit both components with the minimal number of terms that best extracts the signal given its overlap with the foreground. In this second paper, we utilize the spectral constraints derived in the first paper to calculate the full posterior probability distribution of any signal parameter space of choice. The spectral fit provides the starting point for a Markov Chain Monte Carlo (MCMC) engine that samples the signal without traversing the foreground parameter space. At each MCMC step, we marginalize over the weights of all linear foreground modes and suppress those with unimportant variations by applying priors gleaned from the training set. This method drastically reduces the number of MCMC parameters, augmenting the efficiency of exploration, circumvents the need for selecting a minimal number of foreground modes, and allows the complexity of the foreground model to be greatly increased to simultaneously describe many observed spectra without requiring extra MCMC parameters. Using two nonlinear signal models, one based on the Experiment to Detect the Global Epoch-of-Reionization Signature (EDGES) observations and the other on phenomenological frequencies and temperatures of theoretically expected extrema, we demonstrate the success of this methodology by recovering the input parameters from multiple randomly simulated signals at low radio frequencies (10–200 MHz), while rigorously accounting for realistically modeled beam-weighted foregrounds.

The conceptual designs of radio telescopes in Guizhou karst region

The Five-hundred-meter Aperture Spherical Radio Telescope (FAST)—a large-aperture radio telescope constructed in a karst depression—is completing its commissioning. Several studies have been performed using FAST, including the discovery of over 100 pulsars. Nevertheless, the ability of a single telescope is still limited. The construction of other radio telescopes and telescope arrays around FAST will promote its use for new types of observations and help to achieve additional scientific goals. Based on the analyses of the basic properties of the karst depressions in the Guizhou Province—e.g., their morphologies, aperture diameters, and depths—some conceptual designs have been proposed for large-scale radio telescopes and telescopic arrays. They include a drift-scan survey array, a quasi-steerable radio telescope, a tilted radio telescope, and a fixed-reflector, low-frequency spherical radio telescope. The sky coverage and sensitivity have been calculated for each design, and possible science goals have been discussed, including searches for pulsars and fast radio bursts, as well as studies of variable radio sources. By coordinating their observations with FAST, these telescopes will greatly expand the observational capabilities of FAST and yield substantially more science results, especially in the area of time-domain astronomy.

Particle acceleration in low-power hotspots: modelling the broad-band spectral energy distribution

ABSTRACT The acceleration and radiative processes active in low-power radio hotspots are investigated by means of new deep near-infrared (NIR) and optical Very Large Telescope (VLT) observations, complemented with archival, high-sensitivity VLT, radio Very Large Array (VLA), and X-ray Chandra data. For the three studied radio galaxies (3C 105, 3C 195, and 3C 227), we confirm the detection of NIR/optical counterparts of the observed radio hotspots. We resolve multiple components in 3C 227 West and in 3C 105 South and characterize the diffuse NIR/optical emission of the latter. We show that the linear size of this component (≳4 kpc) makes 3C 105 South a compelling case for particles’ re-acceleration in the post-shock region. Modelling of the radio-to-X-ray spectral energy distribution (SED) of 3C 195 South and 3C 227 W1 gives clues on the origin of the detected X-ray emission. In the context of inverse Compton models, the peculiarly steep synchrotron curve of 3C 195 South sets constraints on the shape of the radiating particles’ spectrum that are testable with better knowledge of the SED shape at low (≲GHz) radio frequencies and in X-rays. The X-ray emission of 3C 227 W1 can be explained with an additional synchrotron component originating in compact (<100 pc) regions, such those revealed by radio observations at 22 GHz, provided that efficient particle acceleration (γ ≳ 107) is ongoing. The emerging picture is that of systems in which different acceleration and radiative processes co-exist.

Investigation of E–H mode transition in magnetic-pole-enhanced inductively coupled neon–argon mixture plasma

In this paper, E–H mode transition in magnetic-pole-enhanced inductively coupled neon–argon mixture plasma is investigated in terms of fundamental plasma parameters as a function of argon fraction (0%–100%), operating pressure (1 Pa, 5 Pa, 10 Pa and 50 Pa), and radio frequency (RF) power (5–100 W). An RF compensated Langmuir probe and optical emission spectroscopy are used for the diagnostics of the plasma under study. Owing to the lower ionization potential and higher collision cross-section of argon, when its fraction in the discharge is increased, the mode transition occurs at lower RF power; i.e. for 0% argon and 1 Pa pressure, the threshold power of the E–H mode transition is 65 W, which reduces to 20 W when the argon fraction is increased. The electron density increases with the argon fraction at a fixed pressure, whereas the temperature decreases with the argon fraction. The relaxation length of the low-energy electrons increases, and decreases for high-energy electrons with argon fraction, due to the Ramseur effect. However, the relaxation length of both groups of electrons decreases with pressure due to reduction in the mean free path. The electron energy probability function (EEPF) profiles are non-Maxwellian in E-mode, attributable to the non-local electron kinetics in this mode; however, they evolve to Maxwellian distribution when the discharge transforms to H-mode due to lower electron temperature and higher electron density in H-mode. The tail of the measured EEPFs is found to deplete in both E- and H-modes when the argon fraction in the discharge is increased, because argon has a much lower excitation potential (11.5 eV) than neon (16.6 eV).

Estimation of the Physical Parameters of a CME at High Coronal Heights Using Low-frequency Radio Observations

Abstract Measuring the physical parameters of coronal mass ejections (CMEs), particularly their entrained magnetic field, is crucial for understanding their physics and for assessing their geoeffectiveness. At the moment, only remote sensing techniques can probe these quantities in the corona, the region where CMEs form and acquire their defining characteristics. Radio observations offer the most direct means for estimating the magnetic field when gyrosynchrotron emission is detected. In this work we measure various CME plasma parameters, including its magnetic field, by modeling the gyrosynchrotron emission from a CME. The dense spectral coverage over a wide frequency range provided by the Murchison Widefield Array (MWA) affords a much better spectral sampling than possible before. The MWA images also provide a much higher imaging dynamic range, enabling us to image these weak emissions. Hence we are able to detect radio emission from a CME at larger distances (∼4.73 R ⊙) than have been reported before. The flux densities reported here are among the lowest measured in similar works. Our ability to make extensive measurements on a slow and otherwise unremarkable CME suggests that with the availability of data from the new-generation instruments like the MWA, it should now be possible to make routine, direct detections of radio counterparts of CMEs.

A more detailed look at Galactic magnetic field models: using free–free absorption in HII regions

Context. Cosmic rays (CRs) and the Galactic magnetic field (GMF) are fundamental actors in many processes in the Milky Way. The observed interaction product of these actors is Galactic synchrotron emission integrated over the line of sight (LOS). A comparison to simulations can be made with this tracer using existing GMF models and CR density models. This probes the GMF strength and morphology and the CR density. Aims. Our aim is to provide insight into the Galactic CR density and the distribution and morphology of the GMF strength by exploring and explaining the differences between the simulations and observations of synchrotron intensity. Methods. At low radio frequencies HII regions become opaque due to free–free absorption. Using these HII regions we can measure the synchrotron intensity over a part of the LOS through the Galaxy. The measured intensity per unit path length, that is, the emissivity, for HII regions at different distances, allows us to probe the variation in synchrotron emission not only across the sky but also in the third dimension of distance. Performing these measurements on a large scale is one of the new applications of the window opened by current low-frequency arrays. Using a number of existing GMF models in conjunction with the Galactic CR modeling code GALPROP, we can simulate these synchrotron emissivities. Results. We present an updated catalog, compiled from the literature, of low-frequency absorption measurements of HII regions, their distances, and electron temperatures. We report a simulated emissivity that shows a compatible trend for HII regions that are near the observer. However, we observe a systematically increasing synchrotron emissivity for HII regions that are far from the observer, which is not compatible with the values simulated by the GMF models and GALPROP. Conclusions. Current GMF models plus a GALPROP generated CR density model cannot explain low-frequency absorption measurements. One possibility is that distances to all HII regions catalogued at the kinematic “far” distance are erroneously determined, although this is unlikely since it ignores all evidence for far distances in the literature. However, a detection bias due to the nature of this tracer requires us to keep in mind that certain sources may be missed in an observation. The other possibilities are an enhanced emissivity in the outer Galaxy or a diminished emissivity in the inner Galaxy.

Searching for dark matter signals from local dwarf spheroidal galaxies at low radio frequencies in the GLEAM survey

ABSTRACT The search for emission from weakly interacting massive particle (WIMP) dark matter annihilation and decay has become a multipronged area of research not only targeting a diverse selection of astrophysical objects, but also taking advantage of the entire electromagnetic spectrum. The decay of WIMP particles into standard model particles has been suggested as a possible channel for synchrotron emission to be detected at low radio frequencies. Here, we present the stacking analysis of a sample of 33 dwarf spheroidal (dSph) galaxies with low-frequency (72–231 MHz) radio images from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey. We produce radial surface brightness profiles of images centred upon each dSph galaxy with background radio sources masked. We remove 10 fields from the stacking due to contamination from either poorly subtracted, bright radio sources or strong background gradients across the field. The remaining 23 dSph galaxies are stacked in an attempt to obtain a statistical detection of any WIMP-induced synchrotron emission in these systems. We find that the stacked radial brightness profile does not exhibit a statistically significant detection above the 95 per cent confidence level of ∼1.5 mJy beam−1. This novel technique shows the potential of using low-frequency radio images to constrain fundamental properties of particle dark matter.

Searching for the largest bound atoms in space

Context. Radio recombination lines (RRLs) at frequencies ν < 250 MHz trace the cold, diffuse phase of the interstellar medium, and yet, RRLs have been largely unexplored outside of our Galaxy. Next-generation low-frequency interferometers such as LOFAR, MWA, and the future SKA will, with unprecedented sensitivity, resolution, and large fractional bandwidths, enable the exploration of the extragalactic RRL universe. Aims. We describe methods used to (1) process LOFAR high band antenna (HBA) observations for RRL analysis, and (2) search spectra for RRLs blindly in redshift space. Methods. We observed the radio quasar 3C 190 (z ≈ 1.2) with the LOFAR HBA. In reducing these data for spectroscopic analysis, we placed special emphasis on bandpass calibration. We devised cross-correlation techniques that utilize the unique frequency spacing between RRLs to significantly identify RRLs in a low-frequency spectrum. We demonstrate the utility of this method by applying it to existing low-frequency spectra of Cassiopeia A and M 82, and to the new observations of 3C 190. Results. Radio recombination lines have been detected in the foreground of 3C 190 at z = 1.12355 (assuming a carbon origin) owing to the first detection of RRLs outside of the local universe (first reported in A&A, 622, A7). Toward the Galactic supernova remnant Cassiopeia A, we uncover three new detections: (1) stimulated Cϵ transitions (Δn = 5) for the first time at low radio frequencies, (2) Hα transitions at 64 MHz with a full width at half-maximum of 3.1 km s−1 the most narrow and one of the lowest frequency detections of hydrogen to date, and (3) Cα at vLSR ≈ 0 km s−1 in the frequency range 55–78 MHz for the first time. Additionally, we recover Cα, Cβ, Cγ, and Cδ from the −47 km s−1 and −38 km s−1 components. In the nearby starburst galaxy M 82, we do not find a significant feature. With previously used techniques, we reproduce the previously reported line properties. Conclusions. RRLs have been blindly searched and successfully identified in Galactic (to high-order transitions) and extragalactic (to high redshift) observations with our spectral searching method. Our current searches for RRLs in LOFAR observations are limited to narrow (<100 km s−1) features, owing to the relatively small number of channels available for continuum estimation. Future strategies making use of a wider band (covering multiple LOFAR subbands) or designs with larger contiguous frequency chunks would aid calibration to deeper sensitivities and broader features.

CT performance of compact X-ray source with small focal spot using a 950 keV linear accelerator

A 950 keV X-ray source with small focal spot using a linear accelerator has been developed. By installing a focusing mechanism of electron beam, smaller focal spot (0.3 mm in FWHM of the line spread function) is successfully achieved at 950 keV. 9.3 GHz (X-band) radio frequency is used for accelerating electrons in order to make the size of X-ray source smaller, compared to lower radio frequency such as 3.0 GHz (S-band). Size and weight of the developed X-ray source are W 600 mm × D 800 mm × H 500 mm and 230 kg, respectively. Clearer transmission images using a rectangular groove sample are obtained with small spot setup, compared to conventional one. CT images are also obtained for an engine block, and we demonstrated that advantage of 950 keV X-ray source for CT imaging such as improvement of interior visibility and decrease of artefact, compared to a 450 kV X-ray source. Therefore, we are convinced that this 950 keV X-ray source is widely useful for industrial non-destructive inspection of large structures, because of compactness, high transmission ability and spatial resolution.

The GLEAM 4-Jy (G4Jy) Sample: I. Definition and the catalogue

Abstract The Murchison Widefield Array (MWA) has observed the entire southern sky (Declination, $\delta< 30^{\circ}$ ) at low radio frequencies, over the range 72–231MHz. These observations constitute the GaLactic and Extragalactic All-sky MWA (GLEAM) Survey, and we use the extragalactic catalogue (EGC) (Galactic latitude, $|b| >10^{\circ}$ ) to define the GLEAM 4-Jy (G4Jy) Sample. This is a complete sample of the ‘brightest’ radio sources ( $S_{\textrm{151\,MHz}}>4\,\text{Jy}$ ), the majority of which are active galactic nuclei with powerful radio jets. Crucially, low-frequency observations allow the selection of such sources in an orientation-independent way (i.e. minimising the bias caused by Doppler boosting, inherent in high-frequency surveys). We then use higher-resolution radio images, and information at other wavelengths, to morphologically classify the brightest components in GLEAM. We also conduct cross-checks against the literature and perform internal matching, in order to improve sample completeness (which is estimated to be $>95.5$ %). This results in a catalogue of 1863 sources, making the G4Jy Sample over 10 times larger than that of the revised Third Cambridge Catalogue of Radio Sources (3CRR; $S_{\textrm{178\,MHz}}>10.9\,\text{Jy}$ ). Of these G4Jy sources, 78 are resolved by the MWA (Phase-I) synthesised beam ( $\sim2$ arcmin at 200MHz), and we label 67% of the sample as ‘single’, 26% as ‘double’, 4% as ‘triple’, and 3% as having ‘complex’ morphology at $\sim1\,\text{GHz}$ (45 arcsec resolution). We characterise the spectral behaviour of these objects in the radio and find that the median spectral index is $\alpha=-0.740 \pm 0.012$ between 151 and 843MHz, and $\alpha=-0.786 \pm 0.006$ between 151MHz and 1400MHz (assuming a power-law description, $S_{\nu} \propto \nu^{\alpha}$ ), compared to $\alpha=-0.829 \pm 0.006$ within the GLEAM band. Alongside this, our value-added catalogue provides mid-infrared source associations (subject to 6” resolution at 3.4 $\mu$ m) for the radio emission, as identified through visual inspection and thorough checks against the literature. As such, the G4Jy Sample can be used as a reliable training set for cross-identification via machine-learning algorithms. We also estimate the angular size of the sources, based on their associated components at $\sim1\,\text{GHz}$ , and perform a flux density comparison for 67 G4Jy sources that overlap with 3CRR. Analysis of multi-wavelength data, and spectral curvature between 72MHz and 20GHz, will be presented in subsequent papers, and details for accessing all G4Jy overlays are provided at https://github.com/svw26/G4Jy.

A survey of spatially and temporally resolved radio frequency interference in the FM band at the Murchison Radio-astronomy Observatory

AbstractWe present the first survey of radio frequency interference (RFI) at the future site of the low frequency Square Kilometre Array (SKA), the Murchison Radio-astronomy Observatory (MRO), that both temporally and spatially resolves the RFI. The survey is conducted in a 1 MHz frequency range within the FM band, designed to encompass the closest and strongest FM transmitters to the MRO (located in Geraldton, approximately 300 km distant). Conducted over approximately three days using the second iteration of the Engineering Development Array in an all-sky imaging mode, we find a range of RFI signals. We are able to categorise the signals into: those received directly from the transmitters, from their horizon locations; reflections from aircraft (occupying approximately 13% of the observation duration); reflections from objects in Earth orbit; and reflections from meteor ionisation trails. In total, we analyse 33 994 images at 7.92 s time resolution in both polarisations with angular resolution of approximately 3.5$^{\circ}$, detecting approximately forty thousand RFI events. This detailed breakdown of RFI in the MRO environment will enable future detailed analyses of the likely impacts of RFI on key science at low radio frequencies with the SKA.

ВЛИЯНИЕ СЕЗОННЫХ ВАРИАЦИЙ ПАРАМЕТРОВ ГРУНТА НА ЧУВСТВИТЕЛЬНОСТЬ ЭЛЕМЕНТА АКТИВНОЙ ФАЗИРОВАННОЙ АНТЕННОЙ РЕШЕТКИ РАДИОТЕЛЕСКОПА ГУРТ

ВЛИЯНИЕ СЕЗОННЫХ ВАРИАЦИЙ ПАРАМЕТРОВ ГРУНТА НА ЧУВСТВИТЕЛЬНОСТЬ ЭЛЕМЕНТА АКТИВНОЙ ФАЗИРОВАННОЙ АНТЕННОЙ РЕШЕТКИ РАДИОТЕЛЕСКОПА ГУРТ

Low-frequency Radio Absorption in Tycho’s Supernova Remnant

Abstract Tycho’s supernova remnant (SNR) is the remnant of the SN Ia explosion SN1572. In this work we present new low-frequency radio observations with the LOw Frequency ARray (LOFAR) Low-band and High-band Antennae, centered at 58 MHz and 143 MHz, and with an angular resolution of 41″ and 6″, respectively. We compare these maps to Very Large Array maps at 327 MHz and 1420 MHz, and detect the effect of low-frequency absorption in some regions of the remnant due to the presence of free electrons along the line of sight. We investigate two origins for the low-frequency free–free absorption that we observe: external absorption from the foreground and internal absorption from Tycho’s unshocked ejecta. The external absorption could be due to an ionized thin, diffuse cavity surrounding the SNR (although this cavity would need to be very thin to comply with the neutral fraction required to explain the remnant’s optical lines), or it could be due to an over-ionized molecular shell in the vicinity of the remnant. We note that possible ionizing sources are the X-ray emission from Tycho, its cosmic rays, or radiation from Tycho’s progenitor. For the internal absorption, we are limited by our understanding of the spectral behavior of the region at unabsorbed radio frequencies. However, the observations are suggestive of free–free absorption from unshocked ejecta inside Tycho’s reverse shock.

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.

New Limits on the Low-frequency Radio Transient Sky Using 31 hr of All-sky Data with the OVRO–LWA

Abstract We present the results of the first transient survey from the Owens Valley Radio Observatory Long Wavelength Array (OVRO–LWA) using 31 hr of data, in which we place the most constraining limits on the instantaneous transient surface density at timescales of 13 s to a few minutes and at frequencies below 100 MHz. The OVRO–LWA is a dipole array that images the entire viewable hemisphere with 58 MHz of bandwidth from 27 to 84 MHz at 13 s cadence. No transients are detected above a 6.5σ flux density limit of 10.5 Jy, implying an upper limit to the transient surface density of 2.5 × 10−8 deg−2 at the shortest timescales probed, which is orders of magnitude deeper than has been achieved at sub-100 MHz frequencies and comparable flux densities to date. The nondetection of transients in the OVRO–LWA survey, particularly at minutes-long timescales, allows us to place further constraints on the rate of the potential population of transients uncovered by Stewart et al. From their transient rate, we expect a detection of events, and the probability of our null detection is , ruling out a transient rate >1.4 × 10−4 days−1 deg−2 with 95% confidence at a flux density limit of 18.1 Jy, under the assumption of a flat spectrum and wide bandwidth. We discuss the implications of our nondetection for this population and further constraints that can be made on the source spectral index, intrinsic emission bandwidth, and resulting luminosity distribution.