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  • Thyagarajan, Nithyanandan  (35)
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  • 1
    In: Monthly Notices of the Royal Astronomical Society, 2018, Vol. 477(1), pp.864-866
    Keywords: Errata, Addenda, Cosmology: Observations ; Methods: Statistical ; Dark Ages, Reionization, First Stars
    ISSN: 0035-8711
    E-ISSN: 1365-2966
    Source: Oxford University Press
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  • 2
    In: Monthly Notices of the Royal Astronomical Society, 2017, Vol. 470(4), pp.4720-4731
    Description: We present the E-field Parallel Imaging Calibration (EPICal) algorithm, which addresses the need for a fast calibration method for direct imaging radio astronomy correlators. Direct imaging involves a spatial fast Fourier transform of antenna signals, alleviating an $\mathcal {O}(N_{\mathrm{a}} ^2)$ computational bottleneck typical in radio correlators, and yielding a more gentle $\mathcal {O}(N_{\mathrm{g}} \log _2 N_{\mathrm{g}})$ scaling, where N a is the number of antennas in the array and N g is the number of gridpoints in the imaging analysis. This can save orders of magnitude in computation cost for next generation arrays consisting of hundreds or thousands of antennas. However, because antenna signals are mixed in the imaging correlator without creating visibilities, gain correction must be applied prior to imaging, rather than on visibilities post-correlation. We develop the EPICal algorithm to form gain solutions quickly and without ever forming visibilities. This method scales as the number of antennas, and produces results comparable to those from visibilities. We use simulations to demonstrate the EPICal technique and study the noise properties of our gain solutions, showing they are similar to visibility-based solutions in realistic situations. By applying EPICal to 2 s of Long Wavelength Array data, we achieve a 65 per cent dynamic range improvement compared to uncalibrated images, showing this algorithm is a promising solution for next generation instruments.
    Keywords: Instrumentation: Interferometers ; Techniques: Image Processing ; Techniques: Interferometric
    ISSN: 0035-8711
    E-ISSN: 1365-2966
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  • 3
    In: Monthly Notices of the Royal Astronomical Society, 2017, Vol. 467(1), pp.715-730
    Description: Modern radio telescopes are favouring densely packed array layouts with large numbers of antennas ( N A ≳ 1000). Since the complexity of traditional correlators scales as $\mathcal {O}(N_{\rm A}^2)$ , there will be a steep cost for realizing the full imaging potential of these powerful instruments. Through our generic and efficient E-field Parallel Imaging Correlator ( epic ), we present the first software demonstration of a generalized direct imaging algorithm, namely the Modular Optimal Frequency Fourier imager. Not only does it bring down the cost for dense layouts to $\mathcal {O}(N_{\rm A}\log _2N_{\rm A})$ but can also image from irregular layouts and heterogeneous arrays of antennas. epic is highly modular, parallelizable, implemented in object-oriented python , and publicly available. We have verified the images produced to be equivalent to those from traditional techniques to within a precision set by gridding coarseness. We have also validated our implementation on data observed with the Long Wavelength Array (LWA1). We provide a detailed framework for imaging with heterogeneous arrays and show that epic robustly estimates the input sky model for such arrays. Antenna layouts with dense filling factors consisting of a large number of antennas such as LWA, the Square Kilometre Array, Hydrogen Epoch of Reionization Array, and Canadian Hydrogen Intensity Mapping Experiment will gain significant computational advantage by deploying an optimized version of epic . The algorithm is a strong candidate for instruments targeting transient searches of fast radio bursts as well as planetary and exoplanetary phenomena due to the availability of high-speed calibrated time-domain images and low output bandwidth relative to visibility-based systems.
    Keywords: Instrumentation: Interferometers ; Techniques: Image Processing ; Techniques: Interferometric ; Telescopes
    ISSN: 0035-8711
    E-ISSN: 1365-2966
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  • 4
    Language: English
    In: Monthly Notices of the Royal Astronomical Society, 06/11/2018, Vol.477(1), pp.864-866
    Keywords: Meteorology & Climatology ; Astronomy & Astrophysics;
    ISSN: 0035-8711
    E-ISSN: 1365-2966
    Source: Oxford University Press (via CrossRef)
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  • 5
    In: Monthly Notices of the Royal Astronomical Society, 2017, Vol. 474(4), pp.4487-4499
    Description: We present a baseline sensitivity analysis of the Hydrogen Epoch of Reionization Array (HERA) and its build-out stages to one-point statistics (variance, skewness, and kurtosis) of redshifted 21 cm intensity fluctuation from the Epoch of Reionization (EoR) based on realistic mock observations. By developing a full-sky 21 cm light-cone model, taking into account the proper field of view and frequency bandwidth, utilizing a realistic measurement scheme, and assuming perfect foreground removal, we show that HERA will be able to recover statistics of the sky model with high sensitivity by averaging over measurements from multiple fields. All build-out stages will be able to detect variance, while skewness and kurtosis should be detectable for HERA128 and larger. We identify sample variance as the limiting constraint of the measurements at the end of reionization. The sensitivity can also be further improved by performing frequency windowing. In addition, we find that strong sample variance fluctuation in the kurtosis measured from an individual field of observation indicates the presence of outlying cold or hot regions in the underlying fluctuations, a feature that can potentially be used as an EoR bubble indicator.
    Keywords: Methods: Statistical ; Dark Ages, Reionization, First Stars ; Cosmology: Observations
    ISSN: 0035-8711
    E-ISSN: 1365-2966
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  • 6
    Description: We study the impact of instrumental systematics on the variance, skewness, and kurtosis of redshifted 21 cm intensity fluctuation observations from the Epoch of Reionization. We simulate realistic 21 cm observations based on the Murchison Widefield Array (MWA) Phase I reionization experiment, using the array's point spread function (PSF) and antenna beam patterns, full-sky 21 cm models, and the FHD imaging pipeline. We measure the observed redshift evolution of pixel probability density functions (PDF) and one-point statistics from the simulated maps, comparing them to the measurements derived from simpler simulations that represent the instrument PSFs with Gaussian kernels. We find that both methods yield statistics with similar trends with greater than 80% correlation. We perform additional simulations based on the Hydrogen Epoch of Reionization Array (HERA), using Gaussian kernels as the instrument PSFs, and study the effect of frequency binning on the statistics. We find that PSF smoothing and sampling variance from measuring the statistics over limited field of view dilute intrinsic features and add fluctuations to the statistics but reveal new detectable features. Observed kurtosis will increase when a few extremely high or low temperature regions are present in the maps. Frequency binning reduces the thermal uncertainty but can also blur regions along the frequency dimension, resulting in kurtosis peaks that only appear in statistics derived from maps of certain frequency bins. We further find that the kurtosis peaks will reach their maxima when the angular resolution of the PSFs match the size scale of the extreme regions that produce the peaks. The HERA array should be capable of charting the evolution of the observed skewness and kurtosis of the 21 cm fluctuations with high sensitivity while the MWA Phase I will likely be capable of detecting the peak in variance. Comment: 14 pages, 12 figures, submitted to ApJ
    Keywords: Astrophysics - Cosmology And Nongalactic Astrophysics
    Source: Cornell University
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  • 7
  • 8
    Language: English
    In: Monthly Notices of the Royal Astronomical Society, 2016, Vol.460(4), pp.4320-4347
    Description: We present first results from radio observations with the Murchison Widefield Array seeking to constrain the power spectrum of 21 cm brightness temperature fluctuations between the redshifts of 11.6 and 17.9 (113 and 75 MHz). 3 h of observations were...
    Keywords: Techniques: Interferometric ; Dark Ages ; Reionization ; First Stars ; Radio Lines: General ; X-Rays: Galaxies
    ISSN: 0035-8711
    ISSN: 1365-2966
    Source: NARCIS (National Academic Research and Collaborations Information System)
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  • 9
    Language: English
    In: The Astrophysical Journal Letters, 2015, Vol.807(2), p.L28 (5pp)
    Description: We confirm our recent prediction of the “pitchfork” foreground signature in power spectra of high-redshift 21 cm measurements where the interferometer is sensitive to large-scale structure on all baselines. This is due to the inherent response of a wide-field instrument and is characterized by enhanced power from foreground emission in Fourier modes adjacent to those considered to be the most sensitive to the cosmological H i signal. In our recent paper, many signatures from the simulation that predicted this feature were validated against Murchison Widefield Array (MWA) data, but this key pitchfork signature was close to the noise level. In this paper, we improve the data sensitivity through the coherent averaging of 12 independent snapshots with identical instrument settings and provide the first confirmation of the prediction with a signal-to-noise ratio . This wide-field effect can be mitigated by careful antenna designs that suppress sensitivity near the horizon. Simple models for antenna apertures that have been proposed for future instruments such as the Hydrogen Epoch of Reionization Array and the Square Kilometre Array indicate they should suppress foreground leakage from the pitchfork by ∼40 dB relative to the MWA and significantly increase the likelihood of cosmological signal detection in these critical Fourier modes in the three-dimensional power spectrum.
    Keywords: Astrophysics - Cosmology And Nongalactic Astrophysics ; Astrophysics - Instrumentation And Methods For Astrophysics;
    ISSN: 2041-8205
    E-ISSN: 2041-8213
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  • 10
    Language: English
    In: Experimental Astronomy, 2018, Vol.45(2), pp.177-199
    Description: Spectral structures due to the instrument response is the current limiting factor for the experiments attempting to detect the redshifted 21 cm signal from the Epoch of Reionization (EoR). Recent advances in the delay spectrum methodology for measuring the redshifted 21 cm EoR power spectrum brought new attention to the impact of an antenna’s frequency response on the viability of making this challenging measurement. The delay spectrum methodology provides a somewhat straightforward relationship between the time-domain response of an instrument that can be directly measured and the power spectrum modes accessible to a 21 cm EoR experiment. In this paper, we derive the explicit relationship between antenna reflection coefficient ( S 11 ) measurements made by a Vector Network Analyzer (VNA) and the extent of additional foreground contaminations in delay space. In the light of this mathematical framework, we examine the chromaticity of a prototype antenna element that will constitute the Hydrogen Epoch of Reionization Array (HERA) between 100 and 200 MHz. These reflectometry measurements exhibit additional structures relative to electromagnetic simulations, but we find that even without any further design improvement, such an antenna element will support measuring spatial k modes with line-of-sight components of k ∥ 〉 0.2 h Mpc − 1 . We also find that when combined with the powerful inverse covariance weighting method used in optimal quadratic estimation of redshifted 21 cm power spectra the HERA prototype elements can successfully measure the power spectrum at spatial modes as low as k ∥ 〉 0.1 h Mpc − 1 . This work represents a major step toward understanding the HERA antenna element and highlights a straightforward method for characterizing instrument response for future experiments designed to detect the 21 cm EoR power spectrum.
    Keywords: Astronomical instrumentation ; Methods and techniques ; wideband radio interferometry ; Delay spectrum technique–EoR power spectrum ; 21 cm cosmology
    ISSN: 0922-6435
    E-ISSN: 1572-9508
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