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The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a drift scan radio telescope operating across the 400–800 MHz band. CHIME is located at the Dominion Radio Astrophysical Observatory near Penticton, BC, Canada. The instrument is designed to map neutral hydrogen over the redshift range 0.8–2.5 to constrain the expansion history of the universe. This goal drives the design features of the instrument. CHIME consists of four parallel cylindrical reflectors, oriented north–south, each 100 m × 20 m and outfitted with a 256-element dual-polarization linear feed array. CHIME observes a two-degree-wide stripe covering the entire meridian at any given moment, observing three-quarters of the sky every day owing to Earth’s rotation. An FX correlator utilizes field-programmable gate arrays and graphics processing units to digitize and correlate the signals, with different correlation products generated for cosmological, fast radio burst, pulsar, very long baseline interferometry, and 21 cm absorber back ends. For the cosmology back end, the Nfeed2 correlation matrix is formed for 1024 frequency channels across the band every 31 ms. A data receiver system applies calibration and flagging and, for our primary cosmological data product, stacks redundant baselines and integrates for 10 s. We present an overview of the instrument, its performance metrics based on the first 3 yr of science data, and we describe the current progress in characterizing CHIME’s primary beam response. We also present maps of the sky derived from CHIME data; we are using versions of these maps for a cosmological stacking analysis, as well as for investigation of Galactic foregrounds.

We present a fast and scalable estimator for the binned multifrequency angular bispectrum (MABS) and the 3D bispectrum (BS) of the redshifted 21 cm signal from radio interferometric observations. The estimator operates on gridded visibilities and leverages the fast Fourier transform-based acceleration to efficiently compute the MABS and the 3D BS covering all possible triangle configurations. We present the formalism and validate the estimator using simulated visibility data for a known input model BS, considering the Murchison Widefield Array observations with a bandwidth of 30.72 MHz centered at 154.25 MHz. We consider two cases, namely, without flagging, and with flagging, which has exactly the same frequency channels flagged as the actual data. We obtain estimates of the BS for a wide range of triangle shapes covering the scales 0.003 Mpc−1 ≤ k1 ≤ 1.258 Mpc−1. The estimated BS shows excellent agreement with analytical predictions based on the input model BS. We find that the deviations, which are below 20% even in the presence of flagging, are mostly consistent with the expected statistical fluctuations. This work paves the way for reliable observational estimates of the 21 cm BS for the epoch of reionization, where the signal is predicted to be highly non-Gaussian.

Considering radio-interferometric observations, we present a fast and efficient estimator to compute the binned angular bispectrum (ABS) from gridded visibility data. The estimator makes use of Fast Fourier Transform techniques to compute the ABS covering all possible triangle shapes and sizes. Here, we present the formalism of the estimator and validate it using simulated visibility data for the Murchison Widefield Array observations at ν = 154.25 MHz. We find that our estimator is able to faithfully recover the ABS of the simulated sky signal with ≈10%–15% accuracy for a wide variety of triangle shapes and sizes across the range of angular multipoles 46 ≤ ℓ ≤ 1320. In future work, we plan to apply this to actual data and also generalize it to estimate the three-dimensional redshifted 21 cm bispectrum.

We attempt to measure the z = 8.2 Epoch of Reionization (EoR) 21 cm bispectrum (BS) using Murchison Widefield Array 154.2 MHz data. We find that B(k1⊥, k2⊥, k3⊥, k1∥, k2∥), the 3D cylindrical BS, exhibits a foreground wedge, similar to P(k1⊥, k1∥), the 21 cm cylindrical power spectrum. However, the BS foreground wedge, which depends on (k1⊥, k1∥), (k2⊥, k2∥), and (k3⊥, k3∥), the three sides of a triangle, is more complicated. Considering various foreground avoidance scenarios, we identify the region where all three sides are outside the foreground wedge as the EoR window for the 21 cm BS. However, the EoR window is contaminated by a periodic pattern of spikes that arises from the periodic pattern of missing frequency channels in the data. We evaluate the binned 3D spherical BS for triangles of all possible sizes and shapes, and present results for Δ3, the mean-cube brightness temperature fluctuations. The best 2σ upper limits we obtain for the EoR 21 cm signal are ΔUL3=(1.81×103)3mK3 at k1 = 0.008 Mpc−1 and ΔUL3=(2.04×103)3mK3 at k1 = 0.012 Mpc−1 for equilateral and squeezed triangles, respectively. These are foreground dominated and are many orders of magnitude larger than the predicted EoR 21 cm signal (∼103 mK3).

One major goal in fast radio burst science is to detect fast radio bursts (FRBs) over a wide field of view without sacrificing the angular resolution required to pinpoint them to their host galaxies. Wide-field detection and localization capabilities have already been demonstrated using connected-element interferometry; the CHIME/FRB Outriggers project will push this further using widefield cylindrical telescopes as widefield outriggers for very long baseline interferometry (VLBI). This paper describes an offline VLBI software correlator written in Python for the CHIME/FRB Outriggers project. It includes features well-suited to modern widefield instruments like multibeaming/multiple phase center correlation, pulse gating including coherent dedispersion, and a novel correlation algorithm based on the quadratic estimator formalism. This algorithm mitigates sensitivity loss that arises in instruments where the windowing and channelization is done outside the VLBI correlator at each station, which accounts for a 30% sensitivity drop away from the phase center. Our correlation algorithm recovers this sensitivity on both simulated and real data. As an end-to-end check of our software, we have written a preliminary pipeline for VLBI calibration and single-pulse localization, which we use in Lanman et al. to verify the astrometric accuracy of the CHIME/FRB Outriggers array.

The head–tail (HT) morphology of radio galaxies is seen for a class of radio sources where the primary lobes are being bent in the intercluster weather due to strong interactions between the radio jets and their respective intracluster medium. A systematic search has been carried out for new HT radio galaxies from the Very Large Array Faint Images of the Radio Sky at Twenty-Centimeters survey database at 1400 MHz. Here, we present a catalog of 717 new HT sources, among which 287 are narrow-angle tail (NAT) sources whose opening angle between the two lobes is less than 90°, and 430 are wide-angle tail (WAT) whose the opening angle between the two lobes is greater than 90°. NAT radio sources are characterized by tails bent in a narrow “V”-like shape; the jet bending in the case of WAT radio galaxies are such that the WATs exhibit wide “C”-like morphologies. Optical counterparts are found for 359 HT sources. We report HT sources with luminosity ranges 1038 ≤ L 1.4 GHz ≤ 1045 erg s−1 and redshifts up to 2.01. The various physical properties of these HT sources are mentioned here. Some statistical studies have been done for this large number of newly discovered HT sources.

The 21 cm bispectrum (BS) offers a powerful probe of the Epoch of Reionization (EoR), but its observational access is severely hindered by dominant astrophysical foregrounds. Considering Murchison Widefield Array observations at 154.2 MHz (zc = 8.2), we mitigate foregrounds with smooth component filtering (SCF) and estimate the 21 cm BS. We validate the pipeline using a simulated 21 cm signal and show that the input BS is recovered for modes k∥≥[k∥]f=0.135Mpc−1 . Applied to actual data, the SCF produces substantial foreground suppression, reducing the amplitude of the cylindrical BS B(k1⊥, k2⊥, k3⊥, k1∥, k2∥) by 3–4 orders of magnitude. Artifacts due to missing frequency channels in the data are also suppressed. The resulting EoR window is significantly cleaner at small k⊥. We adopt the region (k1⊥, k2⊥, k3⊥) ≤ 0.026 Mpc−1 and (k1∥, k2∥, k3∥) > 0.135 Mpc−1 to evaluate the 3D spherical BS and constrain the EoR signal. By combining estimates over all triangle shapes, we place the lower and upper limits on the mean cube brightness temperature fluctuations Δ3. The estimates are consistent with statistical fluctuations from system noise. The most stringent lower limit ΔLL3=−(1.25×104)3mK3 and upper limit ΔUL3=(1.22×104)3mK3 are obtained at k1 = 0.281 Mpc−1. Additional observing time will reduce the noise level and enable substantially tighter constraints on the EoR signal.

Correlation for radio interferometer array applications, including Very Long Baseline Interferometry (VLBI), is a multidisciplinary field that traditionally involves astronomy, geodesy, signal processing, and electronic design. In recent years, however, high-performance computing has been taking over electronic design, complicating this mix with the addition of network engineering, parallel programming, and resource scheduling, among others. High-performance applications go a step further by using specialized hardware like Graphics Processing Units (GPUs) or Field Programmable Gate Arrays (FPGAs), challenging engineers to build and maintain high-performance correlators that efficiently use the available resources. Existing literature has generally benchmarked correlators through narrow comparisons on specific scenarios, and the lack of a formal performance characterization prevents a systematic comparison. This combination of ongoing increasing complexity in software correlation together with the lack of performance models in the literature motivates the development of a performance model that allows us not only to characterize existing correlators and predict their performance in different scenarios but, more importantly, to provide an understanding of the trade-offs inherent to the decisions associated with their design. In this paper, we present a model that achieves both objectives. We validate this model against benchmarking results in the literature, and provide an example for its application for improving cost-effectiveness in the usage of cloud resources.

Very long baseline interferometry (VLBI) is an important deep-space tracking technology from radio astronomy. A mission that uses VLBI, China’s lunar sample-return mission Chang’E-5 (CE-5) collects soil samples on the lunar surface, arrives at a rendezvous point, and docks in the lunar orbit through cooperation of the Orbiter and the Ascender. During rendezvous and docking, the Chinese VLBI network (CVN) used same beam VLBI (SBI) to observe two high-velocity probes simultaneously. However, before the launching of the CE-5, CVN lacked the capability to examine dynamic dual-target tracking ability of SBI mode because there were no similar space probes can be used in tests of upgraded VLBI data-processing system. This study proposes a method to simulate VLBI signals and acquire relevant data from the dual-target probe and calibration quasar signals recorded by each station according to the orbits and beacons of probes, station coordinates, etc. The real-time data-processing capability of the system is verified by processing the simulated data during rendezvous and docking. The consistency between simulation and VLBI data-processing was consistent to better than 0.05 ns. This method is universal and can also be used in VLBI simulation and orbital analysis of other deep-space probes.

The digital data produced by each telescope within the observation process of Very Long Baseline Interferometry (VLBI) can be referred to as raw data. The raw telescope data represents the initial data stage (Level 0 data) of the VLBI processing chain, consisting of correlation, fringe-fitting, and geodetic/astrometric parameter estimation. Any systematic effects which are compensated and used for parameter estimation within this chain, are present in the raw telescope data streams. The group delay, although the primary geodetic VLBI observable, can be considered to be the most prominent effect. In this publication, we present a new software package implemented in MATLAB that can simulate raw telescope data for VLBI. The software is called VieRDS ( https://github.com/TUW-VieVS/VieRDS ) and features tools to simulate a delay, a delay rate, a phase offset, and a frequency response of arbitrary magnitude for each channel of the telescope data stream. The signal model consists of a radio source component of selectable flux density, a system noise component, and a phase calibration signal. Furthermore, a tool for one and two bit quantization and a module to store the simulated data in VDIF data format is implemented. For the first time, the simulation of a dispersive group delay for a VGOS broadband frequency setup, and the simulation of the characteristic station frequency response of two VGOS telescopes are shown for demonstration purposes. The simulated data can be correlated and fringe-fitted by commonly used VLBI software correlators and fringe-fitting programs. VieRDS is thus an ideal tool for testing hypotheses of adverse observational effects in a controlled environment by deliberately causing these effects and studying the responses in the correlator output. In this article, we focus on the description of the key characteristic of the simulation concept, the digital signal creation, the digital signal processing algorithms, and the software architecture.