arXiv subjects

Colin J. Lonsdale

Publications and source records attributed to Colin J. Lonsdale.

At least 19 recordsLinked to original sources

arXiv2026

The EDGES Analysis Pipeline: Description and Validation

The sky-averaged redshifted 21-cm signal from Cosmic Dawn is expected to provide a unique view of the first compact objects. However, its measurement remains daunting. Difficulties are driven by the large dynamic contrast between the intervening foregrounds and the signal-of-interest, which places extremely high demands on instrumental calibration and data quality measures. The ongoing debate within the field concerning the evidence of a potential first detection by the EDGES experiment highlights the need for a more robust set of analysis methods and tools that are reliable and accessible. In this paper, we detail for the first time the precise calibration and analysis methodology adopted in previous EDGES data releases. These methods are presented in the context of a new open-source end-to-end analysis and simulation package for 21-cm global signal experiments that both formalizes these methods and provides general tools for the broader community. Finally, we describe the raw data used in previous EDGES papers and release these data publicly for extended scrutiny.

astro-ph.IM↗

arXiv2025

EDGES-3: Instrument Design and Commissioning

EDGES-3 is the third iteration of the EDGES experiment, designed to measure the predicted global absorption feature in the radio spectrum produced by neutral hydrogen gas at cosmic dawn, a critical observation determining when and how the first stars populated the universe. The EDGES-3 instrument has been redesigned to include both the analog and digital electronics within the antenna, allowing for in-situ calibration and removal of the lossy balun found in EDGES-2. EDGES-3 has been on multiple deployments in the past 4 years; to Oregon, Devon Island, Adak Island, and is currently installed and taking data in the outback of Western Australia. This paper provides an accounting of the challenges inherent in the detection of the global, cosmological 21-cm signal, the strategies EDGES employs to mitigate each of these challenges, a description of the instrument, and a report on the Western Australia deployment along with observational data.

astro-ph.IM↗

arXiv2025

A Bayesian approach to modelling spectrometer data chromaticity corrected using beam factors -- II. Model priors and posterior odds

The reliable detection of the global 21-cm signal, a key tracer of Cosmic Dawn and the Epoch of Reionization, requires meticulous data modelling and robust statistical frameworks for model validation and comparison. In Paper I of this series, we presented the Beam-Factor-based Chromaticity Correction (BFCC) model for spectrometer data processed using BFCC to suppress instrumentally induced spectral structure. We demonstrated that the BFCC model, with complexity calibrated by Bayes factor-based model comparison (BFBMC), enables unbiased recovery of a 21-cm signal consistent with the one reported by EDGES from simulated data. Here, we extend the evaluation of the BFCC model to lower amplitude 21-cm signal scenarios where deriving reliable conclusions about a model's capacity to recover unbiased 21-cm signal estimates using BFBMC is more challenging. Using realistic simulations of chromaticity-corrected EDGES-low spectrometer data, we evaluate three signal amplitude regimes -- null, moderate, and high. We then conduct a Bayesian comparison between the BFCC model and three alternative models previously applied to 21-cm signal estimation from EDGES data. To mitigate biases introduced by systematics in the 21-cm signal model fit, we incorporate the Bayesian Null-Test-Evidence-Ratio (BaNTER) validation framework and implement a Bayesian inference workflow based on posterior odds of the validated models. We demonstrate that, unlike BFBMC alone, this approach consistently recovers 21-cm signal estimates that align with the true signal across all amplitude regimes, advancing robust global 21-cm signal detection methodologies.

astro-ph.IM↗

arXiv2025

A general Bayesian model-validation framework based on null-test evidence ratios, with an example application to global 21-cm cosmology

Comparing composite models for multi-component observational data is a prevalent scientific challenge. When fitting composite models, there exists the potential for systematics from a poor fit of one model component to be absorbed by another, resulting in the composite model providing an accurate fit to the data in aggregate but yielding biased a posteriori estimates for individual components. We begin by defining a classification scheme for composite model comparison scenarios, identifying two categories: category I, where models with accurate and predictive components are separable through Bayesian comparison of the unvalidated composite models, and category II, where models with accurate and predictive components may not be separable due to interactions between components, leading to spurious detections or biased signal estimation. To address the limitations of category II model comparisons, we introduce the Bayesian Null Test Evidence Ratio-based (BaNTER) validation framework. Applying this classification scheme and BaNTER to a composite model comparison problem in 21-cm cosmology, where minor systematics from imperfect foreground modelling can bias global 21-cm signal recovery, we validate six composite models using mock data. We show that incorporating BaNTER alongside Bayes-factor-based comparison reliably ensures unbiased inferences of the signal of interest across both categories, positioning BaNTER as a valuable addition to Bayesian inference workflows with potential applications across diverse fields.

astro-ph.CO↗

arXiv2022

Analytic approximations of scattering effects on beam chromaticity in 21-cm global experiments

Scattering from objects near an antenna produce correlated signals from strong compact radio sources in a manner similar to those used by the Sea Interferometer to measure the radio source positions using the fine frequency structure in the total power spectrum of a single antenna. These fringes or ripples due to correlated signal interference are present at a low level in the spectrum of any single antenna and are a major source of systematics in systems used to measure the global redshifted 21-cm signal from the early universe. In the Sea Interferometer a single antenna on a cliff above the sea is used to add the signal from the direct path to the signal from the path reflected from the sea thereby forming an interferometer. This was used for mapping radio sources with a single antenna by Bolton and Slee in the 1950s. In this paper we derive analytic expressions to determine the level of these ripples and compare these results in a few simple cases with electromagnetic modeling software to verify that the analytic calculations are sufficient to obtain the magnitude of the scattering effects on the measurements of the global 21-cm signal. These analytic calculations are needed to evaluate the magnitude of the effects in cases that are either too complex or take too much time to be modeled using software.

astro-ph.IM↗

arXiv2022

Collimation of the relativistic jet in the quasar 3C 273

The collimation of relativistic jets launched from the vicinity of supermassive black holes (SMBHs) at the centers of active galactic nuclei (AGN) is one of the key questions to understand the nature of AGN jets. However, little is known about the detailed jet structure for AGN like quasars since very high angular resolutions are required to resolve these objects. We present very long baseline interferometry (VLBI) observations of the archetypical quasar 3C 273 at 86 GHz, performed with the Global Millimeter VLBI Array, for the first time including the Atacama Large Millimeter/submillimeter Array. Our observations achieve a high angular resolution down to $\sim$60 ${\rm μ}$as, resolving the innermost part of the jet ever on scales of $\sim 10^5$ Schwarzschild radii. Our observations, including close-in-time High Sensitivity Array observations of 3C 273 at 15, 22, and 43 GHz, suggest that the inner jet collimates parabolically, while the outer jet expands conically, similar to jets from other nearby low luminosity AGN. We discovered the jet collimation break around $10^{7}$ Schwarzschild radii, providing the first compelling evidence for structural transition in a quasar jet. The location of the collimation break for 3C 273 is farther downstream the sphere of gravitational influence (SGI) from the central SMBH. With the results for other AGN jets, our results show that the end of the collimation zone in AGN jets is governed not only by the SGI of the SMBH but also by the more diverse properties of the central nuclei.

astro-ph.HE↗

arXiv2020

Space VLBI 2020: Science and Technology Futures Conference Summary

The "Space VLBI 2020: Science and Technology Futures" meeting was the second in The Future of High-Resolution Radio Interferometry in Space series. The first meeting (2018 September 5--6; Noordwijk, the Netherlands) focused on the full range of science applications possible for very long baseline interferometry (VLBI) with space-based antennas. Accordingly, the observing frequencies (wavelengths) considered ranged from below 1~MHz (> 300 m) to above 300~GHz (< 1 mm). For this second meeting, the focus was narrowed to mission concepts and the supporting technologies to enable the highest angular resolution observations at frequencies of 30~GHz and higher (< 1 cm). This narrowing of focus was driven by both scientific and technical considerations. First, results from the RadioAstron mission and the Event Horizon Telescope (EHT) have generated considerable excitement for studying the inner portions of black hole (BH) accretion disks and jets and testing elements of the General Theory of Relativity (GR). Second, the technologies and requirements involved in space-based VLBI differ considerably between 100~MHz and 100~GHz; a related consideration is that there are a number of existing instruments or mission concepts for frequencies of approximately 100~MHz and below, while it has been some time since attention has been devoted to space VLBI at frequencies above 10~GHz. This conference summary attempts to capture elements of presentations and discussions that occurred.

astro-ph.IM↗

arXiv2019

EHT-HOPS pipeline for millimeter VLBI data reduction

We present the design and implementation of an automated data calibration and reduction pipeline for very-long-baseline interferometric (VLBI) observations taken at millimeter wavelengths. These short radio-wavelengths provide the best imaging resolution available from ground-based VLBI networks such as the Event Horizon Telescope (EHT) and the Global Millimeter VLBI Array (GMVA), but require specialized processing due to the strong effects from atmospheric opacity and turbulence as well as the heterogeneous nature of existing global arrays. The pipeline builds upon a calibration suite (HOPS) originally designed for precision geodetic VLBI. To support the reduction of data for astronomical observations, we have developed an additional framework for global phase and amplitude calibration which provides output in a standard data format for astronomical imaging and analysis. We test the pipeline on observations taken at 3.5 mm (86 GHz) by the GMVA joined by the phased Atacama Large Millimeter/submillimeter Array in April 2017, and demonstrate the benefits from the specialized processing of high frequency VLBI data with respect to classical analysis techniques.

astro-ph.IM↗

arXiv2019

Unsupervised generation of high dynamic range solar images: A novel algorithm for self-calibration of interferometry data

Solar radio emission, especially at metre-wavelengths, is well known to vary over small spectral ($\lesssim$100\,kHz) and temporal ($<1$\,s) spans. It is comparatively recently, with the advent of a new generation of instruments, that it has become possible to capture data with sufficient resolution (temporal, spectral and angular) that one can begin to characterize the solar morphology simultaneously along the axes of time and frequency. This ability is naturally accompanied by an enormous increase in data volumes and computational burden, a problem which will only become more acute with the next generation of instruments such as the Square Kilometre Array (SKA). The usual approach, which requires manual guidance of the calibration process, is impractical. Here we present the "Automated Imaging Routine for Compact Arrays for the Radio Sun (AIRCARS)", an end-to-end imaging pipeline optimized for solar imaging with arrays with a compact core. We have used AIRCARS so far on data from the Murchison Widefield Array (MWA) Phase-I. The dynamic range of the images is routinely from a few hundred to a few thousand. In the few cases, where we have pushed AIRCARS to its limits, the dynamic range can go as high as $\sim$75000. The images made represent a substantial improvement in the state-of-the-art in terms of imaging fidelity and dynamic range. This has the potential to transform the multi-petabyte MWA solar archive from raw visibilities into science-ready images. AIRCARS can also be tuned to upcoming telescopes like the SKA, making it a very useful tool for the heliophysics community.

astro-ph.IM↗

arXiv2017

Wavelet-Based Characterization of Small-Scale Solar Emission Features at Low Radio Frequencies

Low radio frequency solar observations using the Murchison Widefield Array have recently revealed the presence of numerous weak, short-lived and narrow-band emission features, even during moderately quiet solar conditions. These non-thermal features occur at rates of many thousands per hour in the 30.72 MHz observing bandwidth, and hence, necessarily require an automated approach for their detection and characterization. Here, we employ continuous wavelet transform using a mother Ricker wavelet for feature detection from the dynamic spectrum. We establish the efficacy of this approach and present the first statistically robust characterization of the properties of these features. In particular, we examine distributions of their peak flux densities, spectral spans, temporal spans and peak frequencies. We can reliably detect features weaker than 1 SFU, making them, to the best of our knowledge, the weakest bursts reported in literature. The distribution of their peak flux densities follows a power law with an index of -2.23 in the 12-155 SFU range, implying that they can provide an energetically significant contribution to coronal and chromospheric heating. These features typically last for 1-2 seconds and possess bandwidths of about 4-5 MHz. Their occurrence rate remains fairly flat in the 140-210 MHz frequency range. At the time resolution of the data, they appear as stationary bursts, exhibiting no perceptible frequency drift. These features also appear to ride on a broadband background continuum, hinting at the likelihood of them being weak type-I bursts.

astro-ph.SR↗

arXiv2016

GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey I: A low-frequency extragalactic catalogue

Using the Murchison Widefield Array (MWA), the low-frequency Square Kilometre Array (SKA1 LOW) precursor located in Western Australia, we have completed the GaLactic and Extragalactic All-sky MWA (GLEAM) survey, and present the resulting extragalactic catalogue, utilising the first year of observations. The catalogue covers 24,831 square degrees, over declinations south of $+30^\circ$ and Galactic latitudes outside $10^\circ$ of the Galactic plane, excluding some areas such as the Magellanic Clouds. It contains 307,455 radio sources with 20 separate flux density measurements across 72--231MHz, selected from a time- and frequency- integrated image centred at 200MHz, with a resolution of $\approx 2$'. Over the catalogued region, we estimate that the catalogue is 90% complete at 170mJy, and 50% complete at 55mJy, and large areas are complete at even lower flux density levels. Its reliability is 99.97% above the detection threshold of $5σ$, which itself is typically 50mJy. These observations constitute the widest fractional bandwidth and largest sky area survey at radio frequencies to date, and calibrate the low frequency flux density scale of the southern sky to better than 10%. This paper presents details of the flagging, imaging, mosaicking, and source extraction/characterisation, as well as estimates of the completeness and reliability. All source measurements and images are available online (http://www.mwatelescope.org/science/gleam-survey). This is the first in a series of publications describing the GLEAM survey results.

astro-ph.GA↗

arXiv2015

Radio Jet Feedback and Star Formation in Heavily Obscured Quasars at Redshifts ~0.3-3, I: ALMA Observations

We present ALMA 870 micron (345 GHz) data for 49 high redshift (0.47<z<2.85), luminous (11.7 < log L(bol) (Lsun) < 14.2) radio-powerful AGN, obtained to constrain cool dust emission from starbursts concurrent with highly obscured radiative-mode black hole (BH) accretion in massive galaxies which possess a small radio jet. The sample was selected from WISE with extremely steep (red) mid-infrared (MIR) colors and with compact radio emission from NVSS/FIRST. Twenty-six sources are detected at 870 microns, and we find that the sample has large mid- to far-infrared luminosity ratios consistent with a dominant and highly obscured quasar. The rest-frame 3 GHz radio powers are 24.7 < log P3.0 GHz (W/Hz) < 27.3, and all sources are radio-intermediate or radio-loud. BH mass estimates are 7.7 < log M(BH) (Msun) < 10.2. The rest frame 1-5 um SEDs are very similar to the "Hot DOGs" (Hot Dust Obscured Galaxies), and steeper (redder) than almost any other known extragalactic sources. ISM masses estimated for the ALMA detected sources are 9.9 < log M(ISM) (Msun) < 11.75 assuming a dust temperature of 30K. The cool dust emission is consistent with star formation rates (SFRs) reaching several thousand Msun/yr, depending on the assumed dust temperature, however we cannot rule out the alternative that the AGN powers all the emission in some cases. Our best constrained source has radiative transfer solutions with ~ equal contributions from an obscured AGN and a young (10-15 Myr) compact starburst.

astro-ph.GA↗

arXiv2015

Real-time imaging of density ducts between the plasmasphere and ionosphere

Ionization of the Earth's atmosphere by sunlight forms a complex, multi-layered plasma environment within the Earth's magnetosphere, the innermost layers being the ionosphere and plasmasphere. The plasmasphere is believed to be embedded with cylindrical density structures (ducts) aligned along the Earth's magnetic field, but direct evidence for these remains scarce. Here we report the first direct wide-angle observation of an extensive array of field-aligned ducts bridging the upper ionosphere and inner plasmasphere, using a novel ground-based imaging technique. We establish their heights and motions by feature-tracking and parallax analysis. The structures are strikingly organized, appearing as regularly-spaced, alternating tubes of overdensities and underdensities strongly aligned with the Earth's magnetic field. These findings represent the first direct visual evidence for the existence of such structures.

physics.space-ph↗

arXiv2015

A Digital-Receiver for the Murchison Widefield Array

An FPGA-based digital-receiver has been developed for a low-frequency imaging radio interferometer, the Murchison Widefield Array (MWA). The MWA, located at the Murchison Radio-astronomy Observatory (MRO) in Western Australia, consists of 128 dual-polarized aperture-array elements (tiles) operating between 80 and 300\,MHz, with a total processed bandwidth of 30.72 MHz for each polarization. Radio-frequency signals from the tiles are amplified and band limited using analog signal conditioning units; sampled and channelized by digital-receivers. The signals from eight tiles are processed by a single digital-receiver, thus requiring 16 digital-receivers for the MWA. The main function of the digital-receivers is to digitize the broad-band signals from each tile, channelize them to form the sky-band, and transport it through optical fibers to a centrally located correlator for further processing. The digital-receiver firmware also implements functions to measure the signal power, perform power equalization across the band, detect interference-like events, and invoke diagnostic modes. The digital-receiver is controlled by high-level programs running on a single-board-computer. This paper presents the digital-receiver design, implementation, current status, and plans for future enhancements.

astro-ph.IM↗

arXiv2014

Study of Redshifted HI from the Epoch of Reionization with Drift scan

The detection of the Epoch of Reionization (EoR) in the redshifted 21-cm line is a challenging task. Here we formulate the detection of the EoR signal using the drift scan strategy. This method potentially has better instrumental stability as compared to the case where a single patch of sky is tracked. We demonstrate that the correlation time between measured visibilities could extend up to 1-2 hr for an interferometer array such as the Murchison Widefield Array (MWA), which has a wide primary beam. We estimate the EoR power based on cross-correlation of visibilities across time and show that the drift scan strategy is capable of the detection of the EoR signal with comparable/better signal-to-noise as compared to the tracking case. We also estimate the visibility correlation for a set of bright point sources and argue that the statistical inhomogeneity of bright point sources might allow their separation from the EoR signal.

astro-ph.IM↗

arXiv2013

A study of fundamental limitations to statistical detection of redshifted HI from the epoch of reionization

In this paper we explore for the first time the relative magnitudes of three fundamental sources of uncertainty, namely, foreground contamination, thermal noise and sample variance in detecting the HI power spectrum from the Epoch of Reionization (EoR). We derive limits on the sensitivity of a Fourier synthesis telescope to detect EoR based on its array configuration and a statistical representation of images made by the instrument. We use the Murchison Widefield Array (MWA) configuration for our studies. Using a unified framework for estimating signal and noise components in the HI power spectrum, we derive an expression for and estimate the contamination from extragalactic point-like sources in three-dimensional k-space. Sensitivity for EoR HI power spectrum detection is estimated for different observing modes with MWA. With 1000 hours of observing on a single field using the 128-tile MWA, EoR detection is feasible (S/N > 1 for $k\lesssim 0.8$ Mpc$^{-1}$). Bandpass shaping and refinements to the EoR window are found to be effective in containing foreground contamination, which makes the instrument tolerant to imaging errors. We find that for a given observing time, observing many independent fields of view does not offer an advantage over a single field observation when thermal noise dominates over other uncertainties in the derived power spectrum.

astro-ph.CO↗

arXiv2013

Science with the Murchison Widefield Array

Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the Southern Hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21 cm emission from the epoch of reionisation in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.

astro-ph.IM↗

arXiv2012

Low Frequency Imaging of Fields at High Galactic Latitude with the Murchison Widefield Array 32-Element Prototype

The Murchison Widefield Array (MWA) is a new low-frequency, wide field-of-view radio interferometer under development at the Murchison Radio-astronomy Observatory (MRO) in Western Australia. We have used a 32-element MWA prototype interferometer (MWA-32T) to observe two 50-degree diameter fields in the southern sky in the 110 MHz to 200 MHz band in order to evaluate the performance of the MWA-32T, to develop techniques for epoch of reionization experiments, and to make measurements of astronomical foregrounds. We developed a calibration and imaging pipeline for the MWA-32T, and used it to produce ~15' angular resolution maps of the two fields. We perform a blind source extraction using these confusion-limited images, and detect 655 sources at high significance with an additional 871 lower significance source candidates. We compare these sources with existing low-frequency radio surveys in order to assess the MWA-32T system performance, wide field analysis algorithms, and catalog quality. Our source catalog is found to agree well with existing low-frequency surveys in these regions of the sky and with statistical distributions of point sources derived from Northern Hemisphere surveys; it represents one of the deepest surveys to date of this sky field in the 110 MHz to 200 MHz band.

astro-ph.CO↗