Direction-dependent Calibration Research Articles

Diffuse radio sky models using large-scale shapelets

Sky models used in radio interferometric data-processing primarily consist of compact and discrete radio sources. When there is a need to model large-scale diffuse structure such as the Galaxy, specialized source models are sought after for the sake of simplicity and computational efficiency. We propose the use of shapelet basis functions for modeling the large-scale diffuse structure in various radio interferometric data-processing pipelines. The conventional source model construction using shapelet basis functions is restricted to using images of smaller size due to limitations in computational resources such as memory. We propose a novel shapelet decomposition method to lift this restriction, enabling the use of images of millions of pixels (as well as a wide spectral bandwidth) for building models of large-scale diffuse structure. Furthermore, the application of direction-dependent errors onto diffuse sky models is an expensive operation that is often performed as a convolution. We propose using some specific properties of shapelet basis functions to apply these direction-dependent errors as a product of the model coefficients, which avoids the need for convolution. We provide results based on simulations and real observations. In order to measure the efficacy of our proposed method in modeling large-scale diffuse structure, we considered the direction-dependent calibration of simulated as well as real LOFAR observations that have a significant number of diffuse large-scale structure. The results show that by including large-scale shapelet models of the diffuse sky, we are able to overcome a major problem of existing calibration techniques, which do not model this large-scale diffuse structure, that is, the suppression of this large-scale diffuse structure because the model is incomplete.

The prototypical major cluster merger Abell 754

Context. Abell 754 is a rich galaxy cluster at z = 0.0543 and is considered the prototype of a major cluster merger. As many dynamically unrelaxed systems, it hosts diffuse radio emission on megaparsec-scales. Extended synchrotron sources in the intra-cluster medium (ICM) are commonly interpreted as evidence that a fraction of the gravitational energy released during cluster mergers is dissipated into nonthermal components. Aims. Here, we aim to use new MeerKAT UHF- and L-band observations to study nonthermal phenomena in Abell 754. These data are complemented with archival XMM-Newton observations to investigate the resolved spectral properties of both the radio and X-ray cluster emission. Methods. For the first time, we employed the pipeline originally developed to calibrate LOFAR data to MeerKAT observations. This allowed us to perform a direction-dependent calibration and obtain highly sensitive radio images in UHF and L bands that capture the extended emission with unprecedented detail. By using a large XMM-Newton mosaic, we produced thermodynamic maps of the ICM. Results. Our analysis reveals that the radio halo in the cluster center is bounded by the well-known shock in the eastern direction. Furthermore, in the southwest periphery, we discover an extended radio source that we classify as a radio relic that is possibly tracing a shock driven by the squeezed gas compressed by the merger, outflowing in perpendicular directions. The low-luminosity of this relic appears compatible with direct acceleration of thermal pool electrons. We interpret the observed radio and X-ray features in the context of a major cluster merger with a nonzero impact parameter. Conclusions. Abell 754 is a remarkable galaxy cluster showcasing exceptional features associated with the ongoing merger event. The high quality of the new MeerKAT data motivates further work on this system.

Ionospheric contributions to the excess power in high-redshift 21-cm power-spectrum observations with LOFAR

ABSTRACT The turbulent ionosphere causes phase shifts to incoming radio waves on a broad range of temporal and spatial scales. When an interferometer is not sufficiently calibrated for the direction-dependent ionospheric effects, the time-varying phase shifts can cause the signal to decorrelate. The ionosphere’s influence over various spatiotemporal scales introduces a baseline-dependent effect on the interferometric array. We study the impact of baseline-dependent decorrelation on high-redshift observations with the Low Frequency Array (LOFAR). Data sets with a range of ionospheric corruptions are simulated using a thin-screen ionosphere model, and calibrated using the state-of-the-art LOFAR epoch of reionization pipeline. For the first time ever, we show the ionospheric impact on various stages of the calibration process including an analysis of the transfer of gain errors from longer to shorter baselines using realistic end-to-end simulations. We find that direction-dependent calibration for source subtraction leaves excess power of up to two orders of magnitude above the thermal noise at the largest spectral scales in the cylindrically averaged autopower spectrum under normal ionospheric conditions. However, we demonstrate that this excess power can be removed through Gaussian process regression, leaving no excess power above the 10 per cent level for a $5~$ km diffractive scale. We conclude that ionospheric errors, in the absence of interactions with other aggravating effects, do not constitute a dominant component in the excess power observed in LOFAR epoch of reionization observations of the North Celestial Pole. Future work should therefore focus on less spectrally smooth effects, such as beam modelling errors.

LOFAR HBA observations of the Euclid Deep Field North (EDFN)

We present the first deep (72 h of observations) radio image of the Euclid Deep Field North (EDFN) obtained with the LOw-Frequency ARray (LOFAR) High Band Antenna (HBA) at 144 MHz. The EDFN is the latest addition to the LOFAR Two-Metre Sky Survey (LoTSS) Deep Fields, and these observations represent the first data release for this field. The observations produced a 6″ resolution image with a central rms noise of 32 μJy beam−1. A catalogue of ~23 000 radio sources above a signal-to-noise ratio threshold of five is extracted from the inner circular 10 deg2 region. We discuss the data analysis, and we provide a detailed description of how we derived the catalogue of radio sources, the issues related to direction-dependent calibration, and their effects on the final products. Finally, we derive the radio source counts at 144 MHz in the EDFN using catalogues of mock radio sources to derive the completeness correction factors. The source counts in the EDFN are consistent with those obtained from the first data release of the other LoTSS Deep Fields (ELAIS-N1, Lockman Hole and Bootes), despite the different method adopted to construct the final catalogue and to assess its completeness.

Bayesian radio interferometric imaging with direction-dependent calibration

Context. Radio interferometers measure frequency components of the sky brightness, modulated by the gains of the individual radio antennas. Due to atmospheric turbulence and variations in the operational conditions of the antennas, these gains fluctuate. Thereby the gains do not only depend on time, but also on the spatial direction on the sky. To recover high-quality radio maps, an accurate reconstruction of the direction and time-dependent individual antenna gains is required. Aims. This paper aims to improve the reconstruction of radio images, by introducing a novel joint imaging and calibration algorithm including direction-dependent antenna gains. Methods. Building on the resolve framework, we designed a Bayesian imaging and calibration algorithm utilizing the image domain gridding method for numerically efficient application of direction-dependent antenna gains. Furthermore, by approximating the posterior probability distribution with variational inference, our algorithm can provide reliable uncertainty maps. Results. We demonstrate the ability of the algorithm to recover high resolution high dynamic range radio maps from VLA data of the radio galaxy Cygnus A. We compare the quality of the recovered images with previous work relying on classically calibrated data. Furthermore, we compare the results with a compressed sensing algorithm also incorporating direction-dependent gains. Conclusions. Including direction-dependent effects in the calibration model significantly improves the dynamic range of the reconstructed images compared to reconstructions from classically calibrated data. Compared to the compressed sensing reconstruction, the resulting sky images have a higher resolution and show fewer artifacts. For utilizing the full potential of radio interferometric data, it is essential to consider the direction dependence of the antenna gains.

LOFAR observations of gravitational wave merger events: O3 results and O4 strategy

ABSTRACT The electromagnetic counterparts to gravitational wave (GW) merger events hold immense scientific value, but are difficult to detect due to the typically large localization errors associated with GW events. The Low-Frequency Array (LOFAR) is an attractive GW follow-up instrument owing to its high sensitivity, large instantaneous field of view, and ability to automatically trigger on events to probe potential prompt emission within minutes. Here, we report on 144-MHz LOFAR radio observations of three GW merger events containing at least one neutron star that were detected during the third GW observing run. Specifically, we probe 9 and 16 per cent of the location probability density maps of S190426c and S200213t, respectively, and place limits at the location of an interesting optical transient (PS19hgw/AT2019wxt) found within the localization map of S191213g. While these GW events are not particularly significant, we use multi-epoch LOFAR data to devise a sensitive wide-field GW follow-up strategy to be used in future GW observing runs. In particular, we improve on our previously published strategy by implementing direction-dependent calibration and mosaicing, resulting in nearly an order of magnitude increase in sensitivity and more uniform coverage. We achieve a uniform 5σ sensitivity of 870 μJy beam−1 across a single instantaneous LOFAR pointing’s 21 deg2 core, and a median sensitivity of 1.1 mJy beam−1 when including the full 89 deg2 hexagonal beam pattern. We also place the deepest transient surface density limits yet on time-scales of the order of month for surveys between 60 and 340 MHz (0.017 deg−2 above 2.0 mJy beam−1 and 0.073 deg−2 above 1.5 mJy beam−1).

Mining mini-halos with MeerKAT I. Calibration and imaging

ABSTRACT Radio mini-halos are clouds of diffuse, low-surface brightness synchrotron emission that surround the Brightest Cluster Galaxy (BCG) in massive cool-core galaxy clusters. In this paper, we use third generation calibration (3GC), also called direction-dependent (DD) calibration, and point source subtraction on MeerKAT extragalactic continuum data. We calibrate and image archival MeerKAT L-band observations of a sample of five galaxy clusters (ACO 1413, ACO 1795, ACO 3444, MACS J1115.8+0129, MACS J2140.2-2339). We use the CARACal pipeline for direction-independent (DI) calibration, DDFacet and killMS for 3GC, followed by visibility-plane point source subtraction to image the underlying mini-halo without bias from any embedded sources. Our 3GC process shows a drastic improvement in artefact removal, to the extent that the local noise around severely affected sources was halved and ultimately resulted in a 7 per cent improvement in global image noise. Thereafter, using these spectrally deconvolved Stokes I continuum images, we directly measure for four mini-halos the flux density, radio power, size, and in-band integrated spectra. Further to that, we show the in-band spectral index maps of the mini-halo (with point sources). We present a new mini-halo detection hosted by MACS J2140.2-2339, having flux density $S_{\rm 1.28\, GHz} = 2.61 \pm 0.31$ mJy, average diameter 296 kpc, and $\alpha ^{\rm 1.5\, GHz}_{\rm 1\, GHz} = 1.21 \pm 0.36$. We also found a ∼100 kpc southern extension to the ACO 3444 mini-halo which was not detected in previous VLA L-band observations. Our description of MeerKAT wide-field, wide-band data reduction will be instructive for conducting further mini-halo science.

Assessing the impact of two independent direction-dependent calibration algorithms on the LOFAR 21 cm signal power spectrum

Context. Detecting the 21 cm signal from the epoch of reionisation (EoR) has been highly challenging due to the strong astrophysical foregrounds, ionospheric effects, radio frequency interference (RFI), and instrumental effects. Better characterisation of their effects and precise calibration are, therefore, crucial for the 21 cm EoR signal detection. Aims. In this work we introduce a newly developed direction-dependent calibration algorithm called DDECAL, and compare its performance with an existing direction-dependent calibration algorithm called SAGECAL, in the context of the LOFAR-EoR 21 cm power spectrum experiment. Methods. We process one night of data from LOFAR observed by the HBA system. The observing frequency ranges between 114 and 127 MHz, corresponding to the redshift from 11.5 and 10.2. The north celestial pole (NCP) and its flanking fields were observed simultaneously in this data set. We analyse the NCP and one of the flanking fields. While the NCP field is calibrated by the standard LOFAR-EoR processing pipeline, using SAGECAL for the direction-dependent calibration with an extensive sky model and 122 directions, for the RA 18h flanking field, DDECAL and SAGECAL are used with a relatively simple sky model and 22 directions. Additionally, two different strategies are used for the subtraction of the very bright and far sources Cassiopeia A and Cygnus A. Results. The resulting estimated 21 cm power spectra show that DDECAL performs better at subtracting sources in the primary beam region, due to the application of a beam model, while SAGECAL performs better at subtracting Cassiopeia A and Cygnus A. The analysis shows that including a beam model during the direction-dependent calibration process significantly improves its overall performance. The benefit is obvious in the primary beam region. We also compare the 21 cm power spectra results on two different fields. The results show that the RA 18h flanking field produces better upper limits compared to the NCP for this particular observation. Conclusions. Despite the minor differences between DDECAL and SAGECAL, due to the beam application, we find that the two algorithms yield comparable 21 cm power spectra on the LOFAR-EoR data after foreground removal. Hence, the current LOFAR-EoR 21 cm power spectrum limits are not likely to depend on the direction-dependent calibration method. For this particular observation, the RA 18h flanking field seems to produce improved upper limits (~30%) compared to the NCP.

Diffuse radio source candidate in CIZA J1358.9−4750

AbstractWe report on results of our upgraded Giant Metrewave Radio Telescope (uGMRT) observations for an early-stage merging galaxy cluster, CIZA J1358.9−4750 (CIZA1359), in Band-3 (300–500 MHz). We achieved the image dynamic range of ∼38000 using the direction dependent calibration and found a candidate of diffuse radio emission at 4σrms significance. The flux density of the candidate at 400 MHz, 24.04 ± 2.48 mJy, is significantly positive compared to noise, where its radio power, 2.40 × 1024 W Hz−1, is consistent with those of typical diffuse radio sources of galaxy clusters. The candidate is associated with a part of the X-ray shock front at which the Mach number reaches its maximum value of $\mathcal {M}\sim 1.7$. The spectral index (Fν ∝ να) of the candidate, α = −1.22 ± 0.33, is in agreement with an expected value derived from the standard diffusive shock acceleration (DSA) model. However, such a low Mach number with a short acceleration time would require seed cosmic rays supplied from active galactic nucleus (AGN) activities of member galaxies, as suggested in some other clusters. Indeed, we found seven AGN candidates inside the diffuse source candidate. Assuming the energy equipartition between magnetic fields and cosmic rays, the magnetic field strength of the candidate was estimated to be 2.1 μG. We also find head–tail galaxies and radio phoenixes or fossils near CIZA1359.

Deep study of A399-401: Application of a wide-field facet calibration

Context. Diffuse synchrotron emission pervades numerous galaxy clusters, indicating the existence of cosmic rays and magnetic fields throughout the intra-cluster medium. The general consensus is that this emission is generated by shocks and turbulence that are activated during cluster merger events and cause a (re-)acceleration of particles to highly relativistic energies. Similar emission has recently been detected in megaparsec-scale filaments connecting pairs of premerging clusters. These instances are the first in which diffuse emission has been found outside of the main cluster regions. Aims. We aim to examine the particle acceleration mechanism in the megaparsec-scale bridge between Abell 399 and Abell 401 and assess in particular whether the synchrotron emission originates from first- or second-order Fermi reacceleration. We also consider the possible influence of active galactic nuclei (AGNs). Methods. To examine the diffuse emission and the AGNs in Abell 399 and Abell 401, we used deep (∼40 h) LOw-Frequency ARray (LOFAR) observations with an improved direction-dependent calibration to produce radio images at 144 MHz with a sensitivity of σ = 79 μJy beam−1 at a 5.9″ × 10.5″ resolution. Using a point-to-point analysis, we searched for a correlation between the radio and X-ray brightness from which we would be able to constrain the particle reacceleration mechanism. Results. Our radio images show the radio bridge between the radio halos at high significance. We find a trend between the radio and X-ray emission in the bridge. We also measured the correlation between the radio and X-ray emission in the radio halos and find a strong correlation for Abell 401 and a weaker correlation for Abell 399. On the other hand, we measure a strong correlation for the radio halo extension from A399 in the northwest direction. With our deep images, we also find evidence for AGN particle injection and reenergized fossil plasma in the radio bridge and halos. Conclusions. We argue that second-order Fermi reacceleration is currently the most favored process to explain the radio bridge. In addition, we find indications for a scenario in which past AGN particle injection might introduce significant scatter in the relation between the radio and X-ray emission in the bridge, but may also supply the fossil plasma needed for in situ reacceleration. The results for Abell 401 are also clearly consistent with a second-order Fermi reacceleration model. The relation between the thermal and nonthermal components in the radio halo in Abell 399 is affected by a recent merger. However, a strong correlation toward its northwest extension and the steep spectrum in the radio halo support an origin of the radio emission in a second-order Fermi reacceleration model as well. The evidence that we find for reenergized fossil plasma near Abell 399 and in the radio bridge supports the reacceleration of the fossil plasma scenario.

Abell 1033: Radio halo and gently reenergized tail at 54 MHz

Context. Abell 1033 is a merging galaxy cluster of moderate mass (M500 = 3.24 × 1014 M⊙). It hosts a broad variety of diffuse radio sources that are linked to different astrophysical phenomena. The most peculiar phenomenon is an elongated feature with an ultra-steep spectrum that is the prototype of the category of gently reenergized tails (GReET). Furthermore, the cluster hosts sources that were previously classified as a radio phoenix and a radio halo. Aims. We aim to improve the understanding of the cosmic-ray acceleration mechanisms in galaxy clusters in a frequency and mass range that has been poorly explored so far. Methods. To investigate the ultra-steep synchrotron emission in the cluster, we performed a full direction-dependent calibration of a LOFAR observation centered at 54 MHz. We analyzed this observation together with recalibrated data of the LOFAR Two-meter Sky Survey at 144 MHz and an archival GMRT observation at 323 MHz. We performed a spectral study of the radio galaxy tail that is connected to the GReET to test whether the current interpretation of the source agrees with observational evidence below 100 MHz. Additionally, we employed a Markov chain Monte Carlo code to fit the halo surface brightness profile at different frequencies. Results. We report an extreme spectral curvature for the GReET. The spectral index flattens from α144 MHz323 MHz ≈ -4 to α144 MHz54 MHz ≈ -2 . This indicates the presence of a cutoff in the electron energy spectrum. At the cluster center, we detect the radio halo at 54, 144, and at lower significance at 323 MHz. We categorize it as an ultra-steep spectrum radio halo with a low-frequency spectral index α = −1.65 ± 0.17. Additionally, with a radio power of P150 MHz = 1.22 ± 0.13 × 1025 W Hz−1, it is found to be significantly above the correlations of radio power to cluster mass reported in the literature. Furthermore, the synchrotron spectrum of the halo is found to further steepen between 144 and 323 MHz, in agreement with the presence of a break in the electron spectrum, which is a prediction of homogeneous reacceleration models.

Statistical analysis of the causes of excess variance in the 21 cm signal power spectra obtained with the Low-Frequency Array

Context.The detection of the 21 cm signal of neutral hydrogen from the Epoch of Reionization (EoR) is challenging due to bright foreground sources, radio frequency interference (RFI), and the ionosphere as well as instrumental effects. Even after correcting for these effects in the calibration step and applying foreground removal techniques, the remaining residuals in the observed 21 cm power spectra are still above the thermal noise, which is referred to as the “excess variance.”Aims.We study a number of potential causes of this excess variance based on 13 nights of data obtained with the Low-Frequency Array (LOFAR).Methods.We focused on the impact of gain errors, the sky model, and ionospheric effects on the excess variance by correlating the relevant parameters such as the gain variance over time or frequency, local sidereal time (LST), diffractive scale, and phase structure–function slope with the level of excess variance.Results.Our analysis shows that the excess variance, at the current level, is neither strongly correlated with gain variance nor the ionospheric parameters. Rather, excess variance has an LST dependence, which is related to the power from the sky. Furthermore, the simulated StokesIpower spectra from bright sources and the excess variance show a similar progression over LST with the minimum power appearing at LST bin 6h to 9h. This LST dependence is also present in sky images of the residual StokesIof the observations. In very-wide sky images based on one night of observation after direction-dependent calibration, we demonstrate that the extra power comes exactly from the direction of bright and distant sources Cassiopeia A and Cygnus A with the array beam patterns.Conclusions.These results suggest that the level of excess variance in the 21 cm signal power spectra is related to sky effects and, hence, it depends on LST. In particular, very bright and distant sources such as Cassiopeia A and Cygnus A can dominate the effect. This is in line with earlier studies and offers a path forward toward a solution, since the correlation between the sky-related effects and the excess variance is non-negligible.

Spatially constrained direction-dependent calibration

ABSTRACT Direction-dependent calibration of widefield radio interferometers estimates the systematic errors, along with multiple directions in the sky. This is necessary because with most systematic errors, which are caused by effects such as the ionosphere or the receiver beam shape, there is a significant spatial variation. Fortunately, there is some deterministic behaviour of these variations in most situations. We enforce this underlying smooth spatial behaviour of systematic errors as an additional constraint on to spectrally constrained direction-dependent calibration. Using both analysis and simulations, we show that this additional spatial constraint improves the performance of multifrequency direction-dependent calibration.

A numerical study of 21-cm signal suppression and noise increase in direction-dependent calibration of LOFAR data

ABSTRACT We investigate systematic effects in direction-dependent gain calibration in the context of the Low-Frequency Array (LOFAR) 21-cm Epoch of Reionization (EoR) experiment. The LOFAR EoR Key Science Project aims to detect the 21-cm signal of neutral hydrogen on interferometric baselines of 50–250 λ. We show that suppression of faint signals can effectively be avoided by calibrating these short baselines using only the longer baselines. However, this approach causes an excess variance on the short baselines due to small gain errors induced by overfitting during calibration. We apply a regularized expectation–maximization algorithm with consensus optimization (sagecal-co) to real data with simulated signals to show that overfitting can be largely mitigated by penalising spectrally non-smooth gain solutions during calibration. This reduces the excess power with about a factor of 4 in the simulations. Our results agree with earlier theoretical analysis of this bias-variance trade off and support the gain-calibration approach to the LOFAR 21-cm signal data.

The LOFAR LBA Sky Survey: Deep Fields

We present the first sub-mJy (≈0.7 mJy beam−1) survey to be completed below 100 MHz, which is over an order of magnitude deeper than previously achieved for widefield imaging of any field at these low frequencies. The high-resolution (15 × 15 arcsec) image of the Boötes field at 34–75 MHz is made from 56 hours of observation with the LOw Frequency ARray (LOFAR) Low Band Antenna (LBA) system. The observations and data reduction, including direction-dependent calibration, are described here. We present a radio source catalogue containing 1948 sources detected over an area of 23.6 deg2, with a peak flux density threshold of 5σ. Using existing datasets, we characterise the astrometric and flux density uncertainties, finding a positional uncertainty of ∼​1.2 arcsec and a flux density scale uncertainty of about 5 per cent. Using the available deep 144-MHz data, we identified 144-MHz counterparts to all the 54-MHz sources, and produced a matched catalogue within the deep optical coverage area containing 829 sources. We calculate the Euclidean-normalised differential source counts and investigate the low-frequency radio source spectral indices between 54 and 144 MHz. Both show a general flattening in the radio spectral indices for lower flux density sources, from ∼ − 0.75 at 144-MHz flux densities between 100 and 1000 mJy to ∼ − 0.5 at 144-MHz flux densities between 5 and 10 mJy. Such flattening is attributable to a growing population of star forming galaxies and compact core-dominated AGN.

MOSS I: Double radio relics in the Saraswati supercluster

ABSTRACT Superclusters are the largest objects in the Universe, and they provide a unique opportunity to study how galaxy clusters are born at the junction of the cosmic web as well as the distribution of magnetic fields and relativistic particles beyond cluster volume. The field of radio astronomy is going through an exciting and important era of the Square Kilometer Array (SKA). We now have the most sensitive functional radio telescopes, such as the MeerKAT, which offers high angular resolution and sensitivity towards diffuse and faint radio sources. To study the radio environments around supercluster, we observed the (core part of) Saraswati supercluster with the MeerKAT. From our MeerKAT Observation of the Saraswati Supercluster (MOSS) project, the initial results of the pilot observations of two massive galaxy clusters, A2631 and ZwCl2341.1+0000, which are located around the dense central part of the Saraswati supercluster, were discussed. In this paper, we describe the observations and data analysis details, including direction-dependent calibration. In particular, we focus on the ZwCl2341.1+0000 galaxy cluster, which hosts double radio relics and puzzling diffuse radio source in the filamentary network. We have imaged these double radio relics in our high resolution and sensitive L-band MeerKAT observation and a puzzling radio source, located between relics, in the low-resolution image. We also derived the spectra of double radio relics using MeerKAT and archival GMRT observations. The following papers will focus on the formation of radio relics and halo, as well as radio galaxy properties in a supercluster core environment.

Investigating ionospheric calibration for LOFAR 2.0 with simulated observations

Context. There are a number of hardware upgrades for the Low-Frequency Array (LOFAR) currently under development. These upgrades are collectively referred to as the LOFAR 2.0 upgrade. The first stage of LOFAR 2.0 will introduce a distributed clock signal and allow for simultaneous observations using all the low-band and high-band antennas of the array. Aims. Our aim is to provide a tool for obtaining accurate simulations for LOFAR 2.0. Methods. We present our software for simulating LOFAR and LOFAR 2.0 observations, which includes realistic models for all important systematic effects such as the first- and second-order ionospheric corruptions, time-variable primary-beam attenuation, station-based delays, and bandpass response. The ionosphere is represented as a thin layer of frozen turbulence. Furthermore, thermal noise can be added to the simulation at the expected level. We simulate a full eight-hour simultaneous low- and high-band antenna observation of a calibrator source and a target field with the LOFAR 2.0 instrument. The simulated data are calibrated using readjusted LOFAR calibration strategies. We examine novel approaches of solution-transfer and joint calibration to improve direction-dependent ionospheric calibration for LOFAR. Results. We find that the calibration of the simulated data behaves very similarly to a real observation and reproduces certain characteristic properties of LOFAR data, such as realistic solutions and image quality. We analyze strategies for direction-dependent calibration of LOFAR 2.0 and find that the ionospheric parameters can be determined most accurately when combining the information of the high-band and low-band in a joint calibration approach. In contrast, the transfer of total electron content solutions from the high-band to the low-band shows good convergence but is highly susceptible to the presence of non-ionospheric phase errors in the data.

Primary beam effects of radio astronomy antennas – II. Modelling MeerKAT L-band beams

ABSTRACT After a decade of design and construction, South Africa’s SKA-MID precursor MeerKAT has begun its science operations. To make full use of the widefield capability of the array, it is imperative that we have an accurate model of the primary beam of its antennas. We have taken available L-band full-polarization ‘astro-holographic’ observations of three antennas and a generic electromagnetic simulation and created sparse representations of the beams using principal components and Zernike polynomials. The spectral behaviour of the spatial coefficients has been modelled using discrete cosine transform. We have provided the Zernike-based model over a diameter of 10 deg averaged over the beams of three antennas in an associated software tool (EIDOS) that can be useful in direction-dependent calibration and imaging. The model is more accurate for the diagonal elements of the beam Jones matrix and at lower frequencies. As we get more accurate beam measurements and simulations in the future, especially for the cross-polarization patterns, our pipeline can be used to create more accurate sparse representations of MeerKAT beams.

VLA imaging of the XMM-LSS/VIDEO deep field at 1–2 GHz

ABSTRACT Modern radio telescopes are routinely reaching depths where normal star-forming galaxies are the dominant observed population. Realizing the potential of radio as a tracer of star formation and black hole activity over cosmic time involves achieving such depths over representative volumes, with radio forming part of a larger multiwavelength campaign. In pursuit of this, we used the Karl G. Jansky Very Large Array (VLA) to image ∼5 deg2 of the VIDEO/XMM-LSS extragalactic deep field at 1–2 GHz. We achieve a median depth of 16 µJy beam−1 with an angular resolution of 4.5 arcsec. Comparisons with existing radio observations of XMM-LSS showcase the improved survey speed of the upgraded VLA: we cover 2.5 times the area and increase the depth by ∼20 per cent in 40 per cent of the time. Direction-dependent calibration and wide-field imaging were required to suppress the error patterns from off-axis sources of even modest brightness. We derive a catalogue containing 5762 sources from the final mosaic. Sub-band imaging provides in-band spectral indices for 3458 (60 per cent) sources, with the average spectrum becoming flatter than the canonical synchrotron slope below 1 mJy. Positional and flux density accuracy of the observations, and the differential source counts are in excellent agreement with those of existing measurements. A public release of the images and catalogue accompanies this article.

Probabilistic direction-dependent ionospheric calibration for LOFAR-HBA

Direction-dependent calibration and imaging is a vital part of producing radio images that are deep and have a high fidelity and highly dynamic range with a wide-field low-frequency array such as LOFAR. Currently, dedicated facet-based direction-dependent calibration algorithms rely on the assumption that the size of the isoplanatic patch is much larger than the separation between bright in-field calibrators. This assumption is often violated owing to the dynamic nature of the ionosphere, and as a result, direction-dependent errors affect image quality between calibrators. In this paper we propose a probabilistic physics-informed model for inferring ionospheric phase screens, providing a calibration for all sources in the field of view. We apply our method to a randomly selected observation from the LOFAR Two-Metre Sky Survey archive, and show that almost all direction-dependent effects between bright calibrators are corrected and that the root-mean-squared residuals around bright sources are reduced by 32% on average.