arXiv subjects

Cathryn M. Trott

Publications and source records attributed to Cathryn M. Trott.

At least 37 records · Page 2Linked to original sources

arXiv2022

Imaging the Southern Sky at 159MHz using Spherical Harmonics with the Engineering Development Array 2

One of the major priorities of international radio astronomy is to study the early universe through the detection of the 21 cm HI line from the epoch of reionisation (EoR). Due to the weak nature of the 21 cm signal, an important part in the detection of the EoR is removing contaminating foregrounds from our observations as they are multiple orders of magnitude brighter. In order to achieve this, sky maps spanning a wide range of frequencies and angular scales are required for calibration and foreground subtraction. Complementing the existing low-frequency sky maps, we have constructed a Southern Sky map through spherical harmonic transit interferometry utilising the engineering development array 2 (EDA2), a square kilometre array (SKA) low-frequency array prototype system. We use the m-mode formalism to create an all-sky map at 159MHz with an angular resolution of 3 degrees, with data from the (EDA2) providing information over +60 degrees to -90 degrees in declination. We also introduce a new method for visualising and quantifying how the baseline distribution of an interferometer maps to the spherical harmonics, and discuss how prior information can be used to constrain spherical harmonic components that the interferometer is not sensitive to.

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arXiv2021

The impact of station far sidelobes on EoR/CD power spectra

Stations of dipole antennas for SKA1-Low will comprise 256 elements spread over an area with a diameter of 38~m. We consider the effect of residual unsubtracted sources well outside of the main beam for differing numbers of unique station configurations, in the Epoch of Reionisation (EoR) and the Cosmic Dawn (CD), both in simulation and with theoretical considerations. We find that beam sidelobes imprint power that renders the cosmological signal unobservable over a range of scales unless compact sources are subtracted beyond θ_Z=30 degrees from zenith, and that station apodization will likely be required to control far sidelobes. An array with N_b=4 unique station configurations is sufficient to reduce the contamination, with an increase to N_b=8 showing little improvement. Comparison with an achromatic Airy disk beam model shows that beam sidelobe level is the main contributor to excess power in the EoR window, and beam chromaticity is less relevant. In the EoR, z=8.5, subtracting sources above 200~mJy out to θ_Z=45 degrees, will be required to access relevant modes of the power spectrum.

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arXiv2021

Constraining the 21cm brightness temperature of the IGM at $z$=6.6 around LAEs with the Murchison Widefield Array

The locations of Ly-$α$ emitting galaxies (LAEs) at the end of the Epoch of Reionisation (EoR) are expected to correlate with regions of ionised hydrogen, traced by the redshifted 21~cm hyperfine line. Mapping the neutral hydrogen around regions with detected and localised LAEs offers an avenue to constrain the brightness temperature of the Universe within the EoR by providing an expectation for the spatial distribution of the gas, thereby providing prior information unavailable to power spectrum measurements. We use a test set of 12 hours of observations from the Murchison Widefield Array (MWA) in extended array configuration, to constrain the neutral hydrogen signature of 58 LAEs, detected with the Subaru Hypersuprime Cam in the \textit{Silverrush} survey, centred on $z$=6.58. We assume that detectable emitters reside in the centre of ionised HII bubbles during the end of reionization, and predict the redshifted neutral hydrogen signal corresponding to the remaining neutral regions using a set of different ionised bubble radii. A prewhitening matched filter detector is introduced to assess detectability. We demonstrate the ability to detect, or place limits upon, the amplitude of brightness temperature fluctuations, and the characteristic HII bubble size. With our limited data, we constrain the brightness temperature of neutral hydrogen to $Δ{\rm T}_B<$30 mK ($<$200 mK) at 95% (99%) confidence for lognormally-distributed bubbles of radii, $R_B =$ 15$\pm$2$h^{-1}$cMpc.

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arXiv2020

The Impact of Tandem Redundant/Sky-Based Calibration in MWA Phase II Data Analysis

Precise instrumental calibration is of crucial importance to 21-cm cosmology experiments. The Murchison Widefield Array's (MWA) Phase II compact configuration offers us opportunities for both redundant calibration and sky-based calibration algorithms; using the two in tandem is a potential approach to mitigate calibration errors caused by inaccurate sky models. The MWA Epoch of Reionization (EoR) experiment targets three patches of the sky (dubbed EoR0, EoR1, and EoR2) with deep observations. Previous work in \cite{Li_2018} and \cite{Wenyang_2019} studied the effect of tandem calibration on the EoR0 field and found that it yielded no significant improvement in the power spectrum over sky-based calibration alone. In this work, we apply similar techniques to the EoR1 field and find a distinct result: the improvements in the power spectrum from tandem calibration are significant. To understand this result, we analyze both the calibration solutions themselves and the effects on the power spectrum over three nights of EoR1 observations. We conclude that the presence of the bright radio galaxy Fornax A in EoR1 degrades the performance of sky-based calibration, which in turn enables redundant calibration to have a larger impact. These results suggest that redundant calibration can indeed mitigate some level of model-incompleteness error.

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arXiv2020

Exploring reionisation and high-z galaxy observables with recent multi-redshift MWA upper limits on the 21-cm signal

We use the latest multi-redshift ($z=6.5-8.7$) upper limits on the 21-cm signal from the Murchison Widefield Array (MWA) to explore astrophysical models which are inconsistent with the data. These upper limits are achieved using 298 h of carefully excised data over four observing seasons. To explore these upper limits in the context of reionisation astrophysics, we use 21CMMC. We then connect the disfavoured regions of parameter space to existing observational constraints on reionisation such as high-$z$ galaxy ultra-violet (UV) luminosity functions, background UV photoionisation rate, intergalactic medium (IGM) neutral fraction, the electron scattering optical depth and the soft-band X-ray emissivity. We find the vast majority of models disfavoured by the MWA limits are already inconsistent with existing observational constraints. These inconsistent models arise from two classes of models: (i) `cold' reionisation and (ii) pure matter density fluctuations (i.e. no reionisation). However, a small subsample of models are consistent implying the existing MWA limits provide unique information in disfavouring models of reionisation, albeit extremely weakly. We also provide the first limits on the soft-band X-ray emissivity from galaxies at high redshifts, finding $1σ$ lower limits of $ε_{{\rm X},0.5-2~{\rm keV}}\gtrsim10^{34.5}$ erg s$^{-1}$ Mpc$^{-3}$. Finally, we recover 95 per cent disfavoured limits on the IGM spin temperature of $\bar{T}_{\rm S}\lesssim$ 1.3, 1.4, 1.5, 1.8, 2.1, 2.4 K at $z=6.5, 6.8, 7.1, 7.8, 8.2, 8.7$. With this we infer the IGM must have undergone, at the very least, a small amount of X-ray heating. Note, the limits on $ε_{{\rm X},0.5-2~{\rm keV}}$ and $\bar{T}_{\rm S}$ are conditional on the IGM neutral fraction.

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arXiv2020

Deep multi-redshift limits on Epoch of Reionisation 21cm Power Spectra from Four Seasons of Murchison Widefield Array Observations

We compute the spherically-averaged power spectrum from four seasons of data obtained for the Epoch of Reionisation (EoR) project observed with the Murchison Widefield Array (MWA). We measure the EoR power spectrum over $k= 0.07-3.0~h$Mpc$^{-1}$ at redshifts $z=6.5-8.7$. The largest aggregation of 110 hours on EoR0 high-band (3,340 observations), yields a lowest measurement of (43~mK)$^2$ = 1.8$\times$10$^3$ mK$^2$ at $k$=0.14~$h$Mpc$^{-1}$ and $z=6.5$ (2$σ$ thermal noise plus sample variance). Using the Real-Time System to calibrate and the CHIPS pipeline to estimate power spectra, we select the best observations from the central five pointings within the 2013--2016 observing seasons, observing three independent fields and in two frequency bands. This yields 13,591 2-minute snapshots (453 hours), based on a quality assurance metric that measures ionospheric activity. We perform another cut to remove poorly-calibrated data, based on power in the foreground-dominated and EoR-dominated regions of the two-dimensional power spectrum, reducing the set to 12,569 observations (419 hours). These data are processed in groups of 20 observations, to retain the capacity to identify poor data, and used to analyse the evolution and structure of the data over field, frequency, and data quality. We subsequently choose the cleanest 8,935 observations (298 hours of data) to form integrated power spectra over the different fields, pointings and redshift ranges.

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arXiv2019

The status of 21cm interferometric experiments

Interferometric experiments of the reionization era offer the advantages of measuring power in spatial modes with increased sensitivity afforded by multiple independent sky measurements. Here we review early work to measure this signal, current experiments, and future opportunities, highlighting the lessons learned along the way that have shaped the research field and experimental design. In particular, this chapter discusses the history, progress, challenges and forecasts for detection and exploration of the spatial structure of the 21~cm brightness temperature signal in the Epoch of Reionisation using interferometric experiments. We discuss GMRT, PAPER, LOFAR, MWA, and the future HERA and SKA.

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arXiv2019

Gridded and direct Epoch of Reionisation bispectrum estimates using the Murchison Widefield Array

We apply two methods to estimate the 21~cm bispectrum from data taken within the Epoch of Reionisation (EoR) project of the Murchison Widefield Array (MWA). Using data acquired with the Phase II compact array allows a direct bispectrum estimate to be undertaken on the multiple redundantly-spaced triangles of antenna tiles, as well as an estimate based on data gridded to the $uv$-plane. The direct and gridded bispectrum estimators are applied to 21 hours of high-band (167--197~MHz; $z$=6.2--7.5) data from the 2016 and 2017 observing seasons. Analytic predictions for the bispectrum bias and variance for point source foregrounds are derived. We compare the output of these approaches, the foreground contribution to the signal, and future prospects for measuring the bispectra with redundant and non-redundant arrays. We find that some triangle configurations yield bispectrum estimates that are consistent with the expected noise level after 10 hours, while equilateral configurations are strongly foreground-dominated. Careful choice of triangle configurations may be made to reduce foreground bias that hinders power spectrum estimators, and the 21~cm bispectrum may be accessible in less time than the 21~cm power spectrum for some wave modes, with detections in hundreds of hours.

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arXiv2019

Robust statistics toward detection of the 21 cm signal from the Epoch of Reionisation

We explore methods for robust estimation of the 21 cm signal from the Epoch of Reionisation (EoR). A Kernel Density Estimator (KDE) is introduced for measuring the spatial temperature fluctuation power spectrum from the EoR. The KDE estimates the underlying probability distribution function of fluctuations as a function of spatial scale, and contains different systematic biases and errors to the typical approach to estimating the fluctuation power spectrum. Extraction of histograms of visibilities allows moments analysis to be used to discriminate foregrounds from 21 cm signal and thermal noise. We use the information available in the histograms, along with the statistical dis-similarity of foregrounds from two independent observing fields, to robustly separate foregrounds from cosmological signal, while making no assumptions about the Gaussianity of the signal. Using two independent observing fields to robustly discriminate signal from foregrounds is crucial for the analysis presented in this paper. We apply the techniques to 13 hours of Murchison Widefield Array (MWA) EoR data over two observing fields. We compare the output to that obtained with a comparative power spectrum estimation method, and demonstrate the reduced foreground contamination using this approach. Using the second moment obtained directly from the KDE distribution functions yields a factor of 2-3 improvement in power for k < 0.3hMpc^{-1} compared with a matched delay space power estimator, while weighting data by additional statistics does not offer significant improvement beyond that available for thermal noise-only weights.

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arXiv2018

The Bias and Uncertainty of Redundant and Sky-Based Calibration under Realistic Sky and Telescope Conditions

The advent of a new generation of low frequency interferometers has opened a direct window into the Epoch of Reionisation (EoR). However, key to a detection of the faint 21-cm signal, and reaching the sensitivity limits of these arrays, is a detailed understanding of the instruments and their calibration. In this work we use simulations to investigate the bias and uncertainty of redundancy based calibration. Specifically, we study the influence of the flux distribution of the radio sky and the impact of antenna position offsets on the complex calibration solutions. We find that the position offsets introduce a bias into the phase component of the calibration solutions. This phase bias increases with the distance between bright radio sources and the pointing center, and with the flux density of these sources. This is potentially problematic for redundant calibration on MWA observations of EoR fields 1 and 2. EoR field 0, however, lacks such sources. We also compared the simulations with theoretical estimates for the bias and uncertainty in sky model based calibration on incomplete sky models for the redundant antenna tiles in the MWA. Our results indicate that redundant calibration outperforms sky based calibration due to the high positional precision of the MWA antenna tiles.

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arXiv2018

The effect of baseline layouts on the EoR foreground wedge: a semi-analytical approach

The 2D power spectrum is a cornerstone of the modern toolkit for analysis of the low-frequency radio interferometric observations of the 21 cm signal arising from the early Universe. Its familiar form disentangles a great deal of systematic information concerning both the sky and telescope, and displays as a foreground-dominated `brick' and `wedge' on large line-of-sight scales, and a complementary `window' on smaller scales. This paper builds on many previous works in the literature which seek to elucidate the varied instrumental and foreground factors which contribute to these familiar structures in the 2D power spectrum. In particular, we consider the effects of $uv$-sampling on the emergence of the wedge. Our results verify the expectation that arbitrarily dense instrument layouts in principal restore the missing information that leads to mode-mixing, and can therefore mitigate the wedge feature. We derive rule-of-thumb estimates for the required baseline density for complete wedge mitigation, showing that these will be unachievable in practice. We also discuss the optimal shape of the layout, showing that logarithmic regularity in the radial separation of baselines is favourable. While complete suppression of foreground leakage into the wedge is practically unachievable, we find that designing layouts which promote radial density and regularity is able to reduce the amplitude of foreground power by 1-3 orders of magnitude.

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arXiv2018

Assessment of ionospheric activity tolerances for Epoch of Reionisation science with the Murchison Widefield Array

Structure imprinted in foreground extragalactic point sources by ionospheric refraction has the potential to contaminate Epoch of Reionisation (EoR) power spectra of the 21~cm emission line of neutral hydrogen. The alteration of the spatial and spectral structure of foreground measurements due to total electron content (TEC) gradients in the ionosphere create a departure from the expected sky signal. We present a general framework for understanding the signatures of ionospheric behaviour in the two-dimensional (2D) neutral hydrogen power spectrum measured by a low-frequency radio interferometer. Two primary classes of ionospheric behaviour are considered, corresponding to dominant modes observed in Murchison Widefield Array (MWA) EoR data; namely, anisotropic structured wave behaviour, and isotropic turbulence. Analytic predictions for power spectrum bias due to this contamination are computed, and compared with simulations. We then apply the ionospheric metric described in Jordan et al. (2017) to study the impact of ionospheric structure on MWA data, by dividing MWA EoR datasets into classes with good and poor ionospheric conditions, using sets of matched 30-minute observations from 2014 September. The results are compared with the analytic and simulated predictions, demonstrating the observed bias in the power spectrum when the ionosphere is active (displays coherent structures or isotropic turbulence). The analysis demonstrates that unless ionospheric activity can be quantified and corrected, active data should not be included in EoR analysis in order to avoid systematic biases in cosmological power spectra. When data are corrected with a model formed from the calibration information, bias reduces below the expected 21~cm signal level. Data are considered `quiet' when the median measured source position offsets are less than 10-15~arcseconds.

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arXiv2018

The Phase II Murchison Widefield Array: Design Overview

We describe the motivation and design details of the "Phase II" upgrade of the Murchison Widefield Array (MWA) radio telescope. The expansion doubles to 256 the number of antenna tiles deployed in the array. The new antenna tiles enhance the capabilities of the MWA in several key science areas. Seventy-two of the new tiles are deployed in a regular configuration near the existing MWA core. These new tiles enhance the surface brightness sensitivity of the MWA and will improve the ability of the MWA to estimate the slope of the Epoch of Reionisation power spectrum by a factor of ~3.5. The remaining 56 tiles are deployed on long baselines, doubling the maximum baseline of the array and improving the array u,v coverage. The improved imaging capabilities will provide an order of magnitude improvement in the noise floor of MWA continuum images. The upgrade retains all of the features that have underpinned the MWA's success (large field-of-view, snapshot image quality, pointing agility) and boosts the scientific potential with enhanced imaging capabilities and by enabling new calibration strategies.

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arXiv2018

Survey parameters for detecting 21cm - Ly$α$ emitter cross correlations with the Square Kilometre Array

Detections of the cross correlation signal between the 21cm signal during reionization and high-redshift Lyman Alpha emitters (LAEs) are subject to observational uncertainties which mainly include systematics associated with radio interferometers and LAE selection. These uncertainties can be reduced by increasing the survey volume and/or the survey luminosity limit, i.e. the faintest detectable Lyman Alpha (Ly$α$) luminosity. We use our model of high-redshift LAEs and the underlying reionization state to compute the uncertainties of the 21cm-LAE cross correlation function at $z\simeq6.6$ for observations with SKA1-Low and LAE surveys with $Δz=0.1$ for three different values of the average IGM ionization state ($\langleχ_\mathrm{HI}\rangle$=0.1, 0.25, 0.5). At $z\simeq6.6$, we find SILVERRUSH type surveys, with a field of view of 21 deg$^2$ and survey luminosity limits of $L_α\geq7.9\times10^{42}$erg~s$^{-1}$, to be optimal to distinguish between an inter-galactic medium (IGM) that is 50%, 25% and 10% neutral, while surveys with smaller fields of view and lower survey luminosity limits, such as the 5 and 10 deg$^2$ surveys with WFIRST, can only discriminate between a 50% and 10% neutral IGM.

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arXiv2018

Source finding in the era of the SKA (precursors): Aegean 2.0

In the era of the SKA precursors, telescopes are producing deeper, larger images of the sky on increasingly small time-scales. The greater size and volume of images place an increased demand on the software that we use to create catalogues, and so our source finding algorithms need to evolve accordingly. In this paper we discuss some of the logistical and technical challenges that result from the increased size and volume of images that are to be analysed, and demonstrate how the Aegean source finding package has evolved to address these challenges. In particular we address the issues of source finding on spatially correlated data, and on images in which the background, noise, and point spread function, vary across the sky. We also introduce the concept of forced or priorized fitting.

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arXiv2018

The Square Kilometre Array Epoch of Reionisation and Cosmic Dawn Experiment

The Square Kilometre Array (SKA) Epoch of Reionisation and Cosmic Dawn (EoR/CD) experiments aim to explore the growth of structure and production of ionising radiation in the first billion years of the Universe. Here I describe the experiments planned for the future low-frequency components of the Observatory, and work underway to define, design and execute these programs.

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arXiv2017

Building models for extended radio sources: implications for Epoch of Reionisation science

We test the hypothesis that limitations in the sky model used to calibrate an interferometric radio telescope, where the model contains extended radio sources, will generate bias in the Epoch of Reionisation (EoR) power spectrum. The information contained in a calibration model about the spatial and spectral structure of an extended source is incomplete because a radio telescope cannot sample all Fourier components. Application of an incomplete sky model to calibration of EoR data will imprint residual error in the data, which propagates forward to the EoR power spectrum. This limited information is studied in the context of current and future planned instruments and surveys at EoR frequencies, such as the Murchison Widefield Array (MWA), Giant Metrewave Radio Telescope (GMRT) and the Square Kilometre Array (SKA1-Low). For the MWA EoR experiment, we find that both the additional short baseline $uv$-coverage of the compact EoR array, and the additional long baselines provided by TGSS and planned MWA expansions, are required to obtain sufficient information on all relevant scales. For SKA1-Low, arrays with maximum baselines of 49~km and 65~km yield comparable performance at 50~MHz and 150~MHz, while 39~km, 14~km and 4~km arrays yield degraded performance.

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arXiv2017

An Improved Statistical Point-Source Foreground Model for the Epoch of Reionization

We present a sophisticated statistical point-source foreground model for low-frequency radio Epoch of Reionization (EoR) experiments using the 21 cm neutral hydrogen emission line. Motivated by our understanding of the low-frequency radio sky, we enhance the realism of two model components compared with existing models: the source count distributions as a function of flux density and spatial position (source clustering), extending current formalisms for the foreground covariance of 2D power spectral modes in 21 cm EoR experiments. The former we generalise to an arbitrarily broken power-law, and the latter to an arbitrary isotropically-correlated field. This paper presents expressions for the modified covariance under these extensions, and shows that for a more realistic source spatial distribution, extra covariance arises in the EoR window which was previously unaccounted for. Failure to include this contribution can yield bias in the final power spectrum and under-estimate uncertainties, potentially leading to a false detection of signal. The extent of this effect is uncertain, owing to ignorance of physical model parameters, but we show that it is dependent on the relative abundance of faint sources, to the effect that our extension will become more important for future deep surveys. Finally, we show that under some parameter choices, ignoring source clustering can lead to false detections on large scales, due to both the induced bias and an artificial reduction in the estimated measurement uncertainty.

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