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  • 1
    Language: English
    In: Nature, December 2018, Vol.564(7736), pp.E35
    Description: [...]their choice to use it over the full band was not justified. Judd D. Bowman1·, Alan E. E. Rogers2, Raul A. Monsalve1'3'4'5'6, Thomas J. Mozdzen1 & Nivedita Mahesh1 1School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA....
    Keywords: United States–Us ; Massachusetts ; Canada ; Quebec Canada ; Arizona ; Chile ; Space Exploration ; Astronomy ; Ionization ; Astronomy ; Calibration ; Astrophysics ; Ionosphere ; Space Exploration ; Parameter Estimation ; Massachusetts Institute of Technology ; Arizona State University ; University of Colorado ; Mcgill University;
    ISSN: 00280836
    E-ISSN: 1476-4687
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  • 2
    Language: English
    In: Nature, Feb 13, 2014, Vol.506(7487), p.163(2)
    Description: Simulations of the cosmos cast doubt on assumptions about the temperature of primordial hydrogen gas when it was ionized by the first stars and galaxies, complicating the interpretation of ongoing observations. SEE LETTER P.197
    Keywords: Hydrogen – Properties ; Ionization – Research ; X-Ray Astronomy – Research ; Simulation – Research
    ISSN: 0028-0836
    E-ISSN: 14764687
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  • 3
    Language: English
    In: Nature, Dec 9, 2010, Vol.468(7325), p.796(3)
    Description: Observations of the 21-centimetre line of atomic hydrogen in the early Universe directly probe the history of the reionization of the gas between galaxies (1). The observations are challenging, though, because of the low expected signal strength (~10 mK), and contamination by strong (〉100K) foreground synchrotron emission in the Milky Way and extragalactic continuum sources (2). If reionization happened rapidly, there should be a characteristic signature (2-4) visible against the smooth foreground in an all-sky spectrum. Here we report an all-sky spectrum between 100 and 200 MHz, corresponding to the redshift range 6 〈 z 〈 13 for the 21-centimetre line. The data exclude a rapid reionization timescale of [DELTA]z 〈 0.06 at the 95% confidence level.
    Keywords: Hydrogen -- Research ; Brightness (Photometry) -- Research ; Radiation (Physics) -- Research
    ISSN: 0028-0836
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  • 4
    In: Nature, 2018, Vol.555(7694), p.67
    Description: After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz. Here we report the detection of a flattened absorption profile in the sky-averaged radio spectrum, which is centred at a frequency of 78 megahertz and has a best-fitting full-width at half-maximum of 19 megahertz and an amplitude of 0.5 kelvin. The profile is largely consistent with expectations for the 21-centimetre signal induced by early stars, however, the best-fitting amplitude of the profile is more than a factor of two greater than the largest predictions. This discrepancy suggests that either the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy, of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude. The low-frequency edge of the observed profile indicates that stars existed and had produced a background of Lyman-alpha photons by 180 million years after the Big Bang. The high-frequency edge indicates that the gas was heated to above the radiation temperature less than 100 million years later. Comment: Accepted version of article published March 1, 2018. Full edited version available through Nature Springer SharedIt at: http://rdcu.be/H0pE
    Keywords: Astrophysics - Cosmology And Nongalactic Astrophysics ; Astrophysics - Instrumentation And Methods For Astrophysics;
    ISSN: 0028-0836
    E-ISSN: 1476-4687
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  • 5
    Language: English
    In: Monthly Notices of the Royal Astronomical Society, Jan, 2012, Vol.419(2), p.1070(15)
    Description: To authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1365-2966.2011.19766.x Byline: Geraint J. A. Harker (12★), Jonathan R. Pritchard (3), Jack O. Burns (12), Judd D. Bowman (4) Keywords: methods: statistical; cosmology: theory; diffuse radiation; radio lines: general Abstract: ABSTRACT Efforts are being made to observe the 21-cm signal from the 'cosmic dawn' using sky-averaged observations with individual radio dipoles. In this paper, we develop a model of the observations accounting for the 21-cm signal, foregrounds and several major instrumental effects. Given this model, we apply Markov Chain Monte Carlo techniques to demonstrate the ability of these instruments to separate the 21-cm signal from foregrounds and quantify their ability to constrain properties of the first galaxies. For concreteness, we investigate observations between 40 and 120 MHz with the proposed Dark Ages Radio Explorer mission in lunar orbit, showing its potential for science return. Author Affiliation: (1)Center for Astrophysics and Space Astronomy, Department of Astrophysics and Planetary Sciences, University of Colorado at Boulder, CO 80309, USA (2)NASA Lunar Science Institute, NASA Ames Research Center, Moffett Field, CA 94035, USA (3)Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA (4)Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287, USA Article History: Accepted 2011 September 2. Received 2011 August 31; in original form 2011 July 14 Article note: (★) E-mail: geraint.harker@colorado.edu
    Keywords: Monte Carlo Methods -- Analysis ; Markov Processes -- Analysis ; Space Exploration -- Analysis ; Universities And Colleges -- Analysis ; Nuclear Radiation -- Analysis
    ISSN: 0035-8711
    Source: Cengage Learning, Inc.
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  • 6
    Language: English
    In: IEEE Transactions on Microwave Theory and Techniques, 2016, Vol.64(8), p.2631(9)
    Keywords: Calibration – Usage ; Network Analyzers – Research ; Transmission Parts – Research
    ISSN: 0018-9480
    Source: Cengage Learning, Inc.
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  • 7
    Language: English
    In: Experimental Astronomy, 2015, Vol.39(1), pp.73-93
    Description: 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.
    Keywords: ADC ; Channelizer ; Digital-receiver ; FPGA ; MWA ; MRO ; PFB ; Radio astronomy instrumentation ; Radio telescope ; SKA
    ISSN: 0922-6435
    E-ISSN: 1572-9508
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  • 8
    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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  • 9
    Language: English
    In: New Astronomy Reviews, 2006, Vol.50(1), pp.173-178
    Description: Statistical epoch of reionization (EOR) observations are one of the emerging fields of experimental cosmology and could provide unique constraints on the formation of structure and the emergence of the first luminous objects. The primary challenge in observing the 21 cm EOR signature is subtracting the strong astrophysical and instrumental foregrounds. In this paper, we review the three-dimensional statistical EOR analysis, present a multi-stage foreground removal framework, and review the design and early field testing for the Mileura Widefield Array.
    Keywords: Cosmology ; Theory ; Epoch of Reionization ; 21 Cm Observations ; Foregrounds ; Astronomy & Astrophysics
    ISSN: 1387-6473
    E-ISSN: 1872-9630
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