Radio Interferometric Measurements Research Articles

Regularized maximum likelihood estimation for radio interferometric imaging in the presence of radiofrequency interferences

We consider a regularized Maximum Likelihood Estimation (MLE) framework to produce images in the context of radio interferometric measurements. Specifically, we consider the class of compound Gaussian distributions to model the additive noise in the presence of radiofrequency interferences. In most cases, direct maximization of the likelihood is not tractable. To overcome this issue, we propose a generic expectation–maximization (EM) algorithm in the presence of a compound Gaussian noise. In addition, we leverage an approximation of the forward radio interferometric operator to derive an original latent data space that allows the use of the FFT in the maximization step, leading to an accelerated extension of the proposed imaging algorithm. The proposed approaches are evaluated on simulated and real data and show a significant improvement in the robustness to the presence of radiofrequency interferences (RFI) in the measurement.

Deep-learning-based radiointerferometric imaging with GAN-aided training

Context. The incomplete coverage of the spatial Fourier space, which leads to imaging artifacts, has been troubling radio interferometry for a long time. The currently best technique is to create an image for which the visibility data are Fourier-transformed and to clean the systematic effects originating from incomplete data in Fourier space. We have shown previously how super-resolution methods based on convolutional neural networks can reconstruct sparse visibility data. Aims. The training data in our previous work were not very realistic. The aim of this work is to build a whole simulation chain for realistic radio sources that then leads to an improved neural net for the reconstruction of missing visibilities. This method offers considerable improvements in terms of speed, automatization, and reproducibility over the standard techniques. Methods. We generated large amounts of training data by creating images of radio galaxies with a generative adversarial network that was trained on radio survey data. Then, we applied the radio interferometer measurement equation in order to simulate the measurement process of a radio interferometer. Results. We show that our neural network can faithfully reconstruct images of realistic radio galaxies. The reconstructed images agree well with the original images in terms of the source area, integrated flux density, peak flux density, and the multiscale structural similarity index. Finally, we show that the neural net can be adapted for estimating the uncertainties in the imaging process.

Electromagnetic emissions in the MHz and GHz frequency ranges driven by the streamer formation processes.

We provide comprehensive data on the spectral and temporal characteristics of low-frequency (LF) (MHz) and high-frequency (HF) (GHz) radio emissions and investigate their correlation with the streamer formation. We show that the propagation of streamers from the cathode is accompanied only by the LF radio emission (10-150MHz). In contrast, the HF radio emission (1-4GHz) arises during the travel of counterstreamers from the anode, which is also indicated by radio interferometric measurements. The power of the LF radio emission sharply increases almost synchronously with that of the HF radio emission. We find that the HF radio emission has a complex spectral and temporal structure and appears as multiple short (less than 1ns) bursts characterized by various frequency components, existing in subnanosecond time intervals.

Fine-grained distributed averaging for large-scale radio interferometric measurement sets

The Square Kilometre Array (SKA) would be the world’s largest radio telescope with eventually over a square kilometre of collecting area. However, there are enormous challenges in its data processing. The use of modern distributed computing techniques to solve the problem of massive data processing in the SKA is one of the most important challenges. In this study, basing on the Dask distribution computational framework, and taking the visibility function integral processing as an example, we adopt a multi-level parallelism method to implement distributed averaging over time and channel. Dask Array was used to implement super large matrix or arrays with supported parallelism. To maximize the usage of memory, we further exploit the data parallelism provided by Dask that intelligently distributes the computational load across a network of computer agents and has a built-in fault tolerance mechanism. The validity of the proposed pattern was also verified by using the Common Astronomy Software Application (CASA), wherein we analyze the smearing effects on images reconstructed from different resolution visibilities.

Efficient wide-field radio interferometry response

Radio interferometers do not measure the sky brightness distribution directly, but measure a modified Fourier transform of it. Imaging algorithms therefore need a computational representation of the linear measurement operator and its adjoint, regardless of the specific chosen imaging algorithm. In this paper, we present a C++ implementation of the radio interferometric measurement operator for wide-field measurements that is based on so-called improved w-stacking. It can provide high accuracy (down to ≈10−12), is based on a new gridding kernel that allows smaller kernel support for given accuracy, dynamically chooses kernel, kernel support, and oversampling factor for maximum performance, uses piece-wise polynomial approximation for cheap evaluations of the gridding kernel, treats the visibilities in cache-friendly order, uses explicit vectorisation if available, and comes with a parallelisation scheme that scales well also in the adjoint direction (which is a problem for many previous implementations). The implementation has a small memory footprint in the sense that temporary internal data structures are much smaller than the respective input and output data, allowing in-memory processing of data sets that needed to be read from disk or distributed across several compute nodes before.

The influence of cumulus clouds on fluctuations in down welling microwave radiation of the atmosphere in the resonance absorption band of water vapor 18...27 GHz

Problem formulating. The nature of fluctuations in the microwave radiation of the atmosphere remains poorly understood. A number of questions, related to the nature of radiation fluctuations near the lines of resonant absorption of various atmospheric components, the intensity of fluctuations at large spatial and temporal intervals requires further study. The influence of various meteorological factors on the intensity of radiation fluctuations requires a deeper consideration. Goal. The goal is to reveal the spectral features of spatio-temporal fluctuations in downward radiation of the atmosphere in the region of the water vapor resonance 18−27 GHz and to estimate the influence of cumulus clouds of various vertical extent on the intensity of fluctuations. Result. Based on the data of long-term measurements of down welling radiation brightness temperature 18−27.2 GHz, the analysis of the spectra of structural function for wide range of temporal intervals is carried out. It is shown that the spectrum of radiation fluctuations in a cloudless atmosphere is similar to the absorption spectrum in water vapor. The spectra of fluctuations in atmospheric radiation under conditions of cumulus cloud cover of various vertical extent are presented. Practical meaning. Radiation fluctuations should be taken into account when creating systems for remote sensing of the atmosphere and the Earth’s surface. They affect the quality of radiometric measurements and radio astronomy observations. The data on fluctuations in atmospheric radiation gathered during this research can be useful for assessing the spatial and temporal fluctuations in the phase delay, for taking into account the influence of atmosphere during the radio interferometric measurements and when observing radiation sources in the diagrammatic modulation mode.

Automated complex “Miсron” for determining measurement error of microwave radio interferometers

The technique of radiointeroferometric measurements is presented and a theoretical estimate of the measuring error the reflecting surface movement is given. The movement are measured with microwave radio interferometers. The automated complex “Miсron” design and principle of operation are described. The complex is intended for experimental determination of the error in measuring the movement of a reflecting surface, as well as for primary and periodic verification of microwave radio interferometers. The complex “Miсron” includes a measuring instrument of the 3rd accuracy class – an incremental linear displacement transducer LIR-7A, which measures the reflecting surface movement of the test object. For the “Miсron” complex verification of the longitudinal movement measuring line is provided, which makes it possible to verify the LIR-7A transducer without dismantling it. Planeparallel gauge blocks or similar standards can be used as the initial standard for verification. The comparison of the movement measured values determined during verification and obtained by an alternative method providing the required accuracy of radio interferometric measurements is carried out.

Primary beam effects of radio astronomy antennas – I. Modelling the Karl G. Jansky Very Large Array (VLA) L-band beam using holography

Modern interferometric imaging relies on advanced calibration that incorporates direction-dependent effects. Their increasing number of antennas (e.g. in LOFAR, VLA, MeerKAT/SKA) and sensitivity are often tempered with the accuracy of their calibration. Beam accuracy drives particularly the capability for high dynamic range imaging (HDR - contrast > 1:$10^6$). The Radio Interferometric Measurement Equation (RIME) proposes a refined calibration framework for wide field of views (i.e. beyond the primary lobe and first null) using beam models. We have used holography data taken on 12 antennas of the Very Large Array (VLA) with two different approaches: a `data-driven' representation derived from Principal Component Analysis (PCA) and a projection on the Zernike polynomials. We determined sparse representations of the beam to encode its spatial and spectral variations. For each approach, we compressed the spatial and spectral distribution of coefficients using low-rank approximations. The spectral behaviour was encoded with a Discrete Cosine Transform (DCT). We compared our modelling to that of the Cassbeam software which provides a parametric model of the antenna and its radiated field. We present comparisons of the beam reconstruction fidelity vs. `compressibility'. We found that the PCA method provides the most accurate model. In the case of VLA antennas, we discuss the frequency ripple over L-band which is associated with a standing wave between antenna reflectors. The results are a series of coefficients that can easily be used `on-the-fly' in calibration pipelines to generate accurate beams at low computing costs.

Reconstructing Video of Time-Varying Sources From Radio Interferometric Measurements

Very long baseline interferometry (VLBI) makes it possible to recover the images of astronomical sources with extremely high angular resolution. Most recently, the Event Horizon Telescope (EHT) has extended VLBI to short millimeter wavelengths with a goal of achieving angular resolution sufficient for imaging the event horizons of nearby supermassive black holes. Interferometry provides measurements related to the underlying source image through a sparse set spatial frequencies. An image can then be recovered from these measurements by making assumptions about the underlying image. One of the most important assumptions made by conventional imaging methods is that over the course of a night's observation the image is static. However, for quickly evolving sources, such as the galactic center's supermassive black hole (SgrA*) targeted by the EHT, this assumption is violated and these conventional imaging approaches fail. This paper presents a new way to model VLBI measurements that allows for the recovery of both the appearance and dynamics of an evolving source by reconstructing a video rather than a static image. By modeling VLBI measurements using a Gaussian Markov Model, information can be propagated across observations in time to reconstruct a video, while simultaneously learning about the dynamics of the source's emission region. This paper demonstrates our proposed expectation-maximization algorithm, StarWarps, on realistic synthetic observations of black holes, and shows how it substantially improves the results compared to conventional imaging methods. In addition to synthetic data, the technique is demonstrated on real VLBA data of the M87 jet.

Radio Interferometric Observation of an Asteroid Occultation

Abstract The occultation of the radio galaxy 0141+268 by the asteroid (372) Palma on 2017 May 15 was observed using six antennas of the Very Long Baseline Array (VLBA). The shadow of Palma crossed the VLBA station at Brewster, Washington. Owing to the wavelength used, and the size and the distance of the asteroid, a diffraction pattern in the Fraunhofer regime was observed. The measurement retrieves both the amplitude and the phase of the diffracted electromagnetic wave. This is the first astronomical measurement of the phase shift caused by diffraction. The maximum phase shift is sensitive to the effective diameter of the asteroid. The bright spot at the shadow’s center, the so called Arago–Poisson spot, is clearly detected in the amplitude time-series, and its strength is a good indicator of the closest angular distance between the center of the asteroid and the radio source. A sample of random shapes constructed using a Markov chain Monte Carlo algorithm suggests that the silhouette of Palma deviates from a perfect circle by 26 ± 13%. The best-fitting random shapes resemble each other, and we suggest their average approximates the shape of the silhouette at the time of the occultation. The effective diameter obtained for Palma, 192.1 ± 4.8 km, is in excellent agreement with recent estimates from thermal modeling of mid-infrared photometry. Finally, our computations show that because of the high positional accuracy, a single radio interferometric occultation measurement can reduce the long-term ephemeris uncertainty by an order of magnitude.

Constraining neutrino mass using the large-scale H i distribution in the post-reionization epoch

The neutral intergalactic medium in the post reionization epoch allows us to study cosmological structure formation through the observation of the redshifted 21 cm signal and the Lyman-alpha forest. We investigate the possibility of measuring the total neutrino mass through the suppression of power in the matter power spectrum. We investigate the possibility of measuring the neutrino mass through its imprint on the cross-correlation power spectrum of the 21-cm signal and the Lyman-alpha forest. We consider a radio-interferometric measurement of the 21 cm signal with a SKA1-mid like radio telescope and a BOSS like Lyman-alpha forest survey. A Fisher matrix analysis shows that at the fiducial redshift z = 2.5, a 10,000 hrs 21-cm observation distributed equally over 25 radio pointings and a Lyman-alpha forest survey with 30 quasars lines of sights in 1 {deg}^2, allows us to measure \Omega_{\nu} at a 3.25 % level. A total of 25,000 hrs radio-interferometric observation distributed equally over 25 radio pointings and a Lyman-alpha survey with \bar n = 60 {\rm deg}^{-2} will allow \Omega_{\nu} to be measured at a 2.26 % level. This corresponds to an idealized measurement of \sum m_{\nu} at the precision of (100 \pm 2.26) meV and f_{\nu}= \Omega_{\nu}/ \Omega_{m} at 2.49 % level.

Radio-interferometric tracking using fault-tolerant phase unwrapping

A novel radio-interferometric measurement method for tracking applications is proposed. Fixed radio transceiver nodes are used to generate interferometric signals, allowing the tracking of a moving object equipped with a radio receiver. Low-frequency phase measurements of the interferometric signals are processed by the proposed simultaneous phase unwrapping algorithm, and then the real-time position estimates are calculated. The performance of the tracking system is analyzed by simulations and real measurements, indicating that the proposed method tolerates well substantial measurement phase noise and even sporadic large measurement errors.

Constraining the parameters of the putative supermassive binary black hole in PG 1302–102 from its radio structure

We investigate the pc-scale kinematics and kpc-scale radio morphology of the quasar PG 1302-102, which may harbour a sub-pc separation supermassive binary black hole system at its centre as inferred from optical variability. High-resolution radio interferometric measurements obtained with the Very Long Baseline Array (VLBA) in the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) programme at 15 GHz at 20 epochs spanning 17 years were analysed to investigate the pc-scale radio structure. Archival observations with the Very Large Array (VLA) at 1.4 GHz and 5 GHz were obtained to study the kpc-scale morphology. We find that the pc-scale jet is inclined within ~2.2 deg to the line of sight and has a half-opening angle of about 0.2 deg. The parameters derived from the pc-scale radio jet are qualitatively consistent with those obtained from the analysis of the optical light curve of PG 1302-102. We obtain at least 0.08 for the mass ratio of the two black holes in the system. We find some indication for a helical jet structure on kpc-scale, but the directions of the inner and the extended radio jets are significantly different, obstructing a straightforward connection of the pc- and kpc-scale jets within the binary scenario.

Montblanc : GPU accelerated radio interferometer measurement equations in support of Bayesian inference for radio observations

Abstract We present Montblanc, a GPU implementation of the Radio interferometer measurement equation (RIME) in support of the Bayesian inference for radio observations (BIRO) technique. BIRO uses Bayesian inference to select sky models that best match the visibilities observed by a radio interferometer. To accomplish this, BIRO evaluates the RIME multiple times, varying sky model parameters to produce multiple model visibilities. χ 2 values computed from the model and observed visibilities are used as likelihood values to drive the Bayesian sampling process and select the best sky model. As most of the elements of the RIME and χ 2 calculation are independent of one another, they are highly amenable to parallel computation. Additionally, Montblanc caters for iterative RIME evaluation to produce multiple χ 2 values. Modified model parameters are transferred to the GPU between each iteration. We implemented Montblanc as a Python package based upon NVIDIA’s CUDA architecture. As such, it is easy to extend and implement different pipelines. At present, Montblanc supports point and Gaussian morphologies, but is designed for easy addition of new source profiles. Montblanc’s RIME implementation is performant: On an NVIDIA K40, it is approximately 250 times faster than MeqTrees on a dual hexacore Intel E5-2620v2 CPU. Compared to the OSKAR simulator’s GPU-implemented RIME components it is 7.7 and 12 times faster on the same K40 for single and double-precision floating point respectively. However, OSKAR’s RIME implementation is more general than Montblanc’s BIRO-tailored RIME. Theoretical analysis of Montblanc’s dominant CUDA kernel suggests that it is memory bound. In practice, profiling shows that is balanced between compute and memory, as much of the data required by the problem is retained in L1 and L2 caches.

Generalized formalisms of the radio interferometer measurement equation

The Radio Interferometer Measurement Equation (RIME) is a matrix-based mathematical model that describes the response of a radio interferometer. The Jones calculus it employs is not suitable for describing the analogue components of a telescope. This is because it does not consider the effect of impedance mismatches between components. This paper aims to highlight the limitations of Jones calculus, and suggests some alternative methods that are more applicable. We reformulate the RIME with a different basis that includes magnetic and mixed coherency statistics. We present a microwave network inspired 2N-port version of the RIME, and a tensor formalism based upon the electromagnetic tensor from special relativity. We elucidate the limitations of the Jones-matrix-based RIME for describing analogue components. We show how measured scattering parameters of analogue components can be used in a 2N-port version of the RIME. In addition, we show how motion at relativistic speed affects the observed flux. We present reformulations of the RIME that correctly account for magnetic field coherency. These reformulations extend the standard formulation, highlight its limitations, and may have applications in space-based interferometry and precise absolute calibration experiments.

Two in one? A possible dual radio-emitting nucleus in the quasar SDSS J1425+3231

The radio-emitting quasar SDSS J1425+3231 (z=0.478) was recently found to have double-peaked narrow [O III] optical emission lines. Based on the analysis of the optical spectrum, Peng et al. (2011) suggested that this object harbours a dual active galactic nucleus (AGN) system, with two supermassive black holes (SMBHs) separated on the kpc scale. SMBH pairs should be ubiquitous according to hierarchical galaxy formation scenarios in which the host galaxies and their central black holes grow together via interactions and eventual mergers. Yet the number of presently-confirmed dual SMBHs on kpc or smaller scales remains small. A possible way to obtain direct observational evidence for duality is to conduct high-resolution radio interferometric measurements, provided that both AGN are in an evolutionary phase when some activity is going on in the radio. We used the technique of Very Long Baseline Interferometry (VLBI) to image SDSS J1425+3231. Observations made with the European VLBI Network (EVN) at 1.7 GHz and 5 GHz frequencies in 2011 revealed compact radio emission at sub-mJy flux density levels from two components with a projected linear separation of \sim2.6 kpc. These two components support the possibility of a dual AGN system. The weaker component remained undetected at 5 GHz, due to its steep radio spectrum. Further study will be necessary to securely rule out a jet--shock interpretation of the less dominant compact radio source. Assuming the dual AGN interpretation, we discuss black hole masses, luminosities, and accretion rates of the two components, using available X-ray, optical, and radio data. While high-resolution radio interferometric imaging is not an efficient technique to search blindly for dual AGN, it is an invaluable tool to confirm the existence of selected candidates.

Revisiting the radio interferometer measurement equation

The radio interferometer measurement equation (RIME), especially in its 2x2 form, has provided a comprehensive matrix-based formalism for describing classical radio interferometry and polarimetry, as shown in the previous three papers of this series. However, recent practical and theoretical developments, such as phased array feeds (PAFs), aperture arrays (AAs) and wide-field polarimetry, are exposing limitations of the formalism. This paper aims to develop a more general formalism that can be used to both clearly define the limitations of the matrix RIME, and to describe observational scenarios that lie outside these limitations. Some assumptions underlying the matrix RIME are explicated and analysed in detail. To this purpose, an array correlation matrix (ACM) formalism is explored. This proves of limited use; it is shown that matrix algebra is simply not a sufficiently flexible tool for the job. To overcome these limitations, a more general formalism based on tensors and the Einstein notation is proposed and explored both theoretically, and with a view to practical implementations. The tensor formalism elegantly yields generalized RIMEs describing beamforming, mutual coupling, and wide-field polarimetry in one equation. It is shown that under the explicated assumptions, tensor equations reduce to the 2x2 RIME. From a practical point of view, some methods for implementing tensor equations in an optimal way are proposed and analysed. The tensor RIME is a powerful means of describing observational scenarios not amenable to the matrix RIME. Even in cases where the latter remains applicable, the tensor formalism can be a valuable tool for understanding the limits of such applicability.

Revisiting the radio interferometer measurement equation

Paper I of the series re-derived the radio interferometry measurement equation (RIME) from first principles, and extended the Jones formalism to the full-sky case, incorporating direction-dependent effects (DDEs). This paper aims to describe both classical radio interferometric calibration (selfcal and related methods), and the recent developments in the treatment of DDEs, using the RIME-based mathematical framework developed in Paper I. It also aims to demonstrate the ease with which the various effects can be described and understood. The first section of this paper uses the RIME formalism to describe self-calibration, both with a full RIME, and with the approximate equations of older software packages, and shows how this is affected by DDEs. The second section gives an overview of real-life DDEs and proposed methods of dealing with them. This results in a formal RIME-based description and comparison of existing and proposed approaches to the problem of DDEs.

Revisiting the radio interferometer measurement equation

Papers I and II of this series have extended the radio interferometry measurement equation (RIME) formalism to the full-sky case, and provided a RIME-based description of calibration and the problem of direction-dependent effects (DDEs). This paper aims to provide a practical demonstration of a RIME-based approach to calibration, via an example of extremely high-dynamic range calibration of WSRT observations of 3C 147 at 21 cm, with full treatment of DDEs. A version of the RIME incorporating differential gains has been implemented in MeqTrees, and applied to the 3C 147 data. This was used to perform regular selfcal, then solve for interferometer-based errors and for differential gains. The resulting image of the field around 3C 147 is thermal noise-limited, has a very high dynamic range (1.6 million), and none of the off-axis artefacts that plague regular selfcal. The differential gain solutions show a high signal-to-noise ratio, and may be used to extract information on DDEs and errors in the sky model. The differential gain approach can eliminate DDE-related artefacts, and provide information for iterative improvements of sky models. Perhaps most importantly, sources as faint as 2 mJy have been shown to yield meaningful differential gain solutions, and thus can be used as potential calibration beacons in other DDE-related schemes.

Revisiting the radio interferometer measurement equation

Since its formulation by Hamaker et al., the radio interferometer measurement equation (RIME) has provided a rigorous mathematical basis for the development of novel calibration methods and techniques, including various approaches to the problem of direction-dependent effects (DDEs). This series of papers aims to place recent developments in the treatment of DDEs into one RIME-based mathematical framework, and to demonstrate the ease with which the various effects can be described and understood. It also aims to show the benefits of a RIME-based approach to calibration. Paper I re-derives the RIME from first principles, extends the formalism to the full-sky case, and incorporates DDEs. Paper II then uses the formalism to describe self-calibration, both with a full RIME, and with the approximate equations of older software packages, and shows how this is affected by DDEs. It also gives an overview of real-life DDEs and proposed methods of dealing with them. Applying this to WSRT data (Paper III) results in a noise-limited image of the field around 3C 147 with a very high dynamic range (1.6 million), and none of the off-axis artifacts that plague regular selfcal. The resulting differential gain solutions contain significant information on DDEs, and can be used for iterative improvements of sky models. Perhaps most importantly, sources as faint as 2 mJy have been shown to yield meaningful differential gain solutions, and thus can be used as potential calibration beacons in other DDE-related schemes.