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  • Bowman, Judd  (72)
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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
    In: Nature, 2010, Vol.468(7325), p.796
    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 galaxies1. The observations are challenging, though, because of the low expected signal strength (~10 mK), and contamination by strong (〉100 K) foreground synchrotron emission in the Milky Way and extragalactic continuum sources2. If reionization happened rapidly, there should be a characteristic signature2, 3, 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 Δz 〈 0.06 at the 95% confidence level.
    Keywords: Sciences (General) ; Physics;
    ISSN: 0028-0836
    E-ISSN: 14764687
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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
    In: Nature, Volume 468, Issue 7325, pp. 796-798 (2010)
    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. The observations are challenging, though, because of the low expected signal strength (~10 mK), and contamination by strong (〉100 K) foreground synchrotron emission in the Milky Way and extragalactic continuum sources2. If reionization happened rapidly, there should be a characteristic signature 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 dz 〈 0.06 at the 95% confidence level. Comment: 8 pages, 2 figures, Published in Nature, Volume 468, Issue 7325, pp. 796-798 (2010)
    Keywords: Astrophysics - Cosmology And Nongalactic Astrophysics
    Source: Cornell University
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  • 6
    In: Radio Science, December 2012, Vol.47(6), pp.n/a-n/a
    Description: A new method of absolute calibration of sky noise temperature using a three‐position switched spectrometer, measurements of antenna and low noise amplifier impedance with a vector network analyzer, and ancillary measurements of the amplifier noise waves is described and compared with other methods of calibration. An initial test of the method has been made using brief observations from 55 to 110 MHz at West Forks, Maine, to estimate the sky noise spectral index. Estimates are made of the accuracy that might ultimately be achieved with observations to detect or set limits on the red‐shifted 21 cm line at a radio quiet site. It is concluded that an antenna reflection coefficient better than about −20 dB is required to avoid being limited by the accuracy of the antenna reflection coefficient measurements using a vector network analyzer. Absolute calibration of antenna and spectrometer for sky noise measurements
    Keywords: Antenna ; Calibration ; Spectrometer
    ISSN: 0048-6604
    E-ISSN: 1944-799X
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  • 7
    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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  • 8
    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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  • 9
    Language: English
    In: IEEE Transactions on Microwave Theory and Techniques, August 2016, Vol.64(8), pp.2631-2639
    Description: This paper introduces a one-port method for estimating model parameters of vector network analyzer calibration standards. The method involves measuring the standards through an asymmetrical passive network connected in direct mode and then in reverse mode, and using these measurements to compute the S-parameters of the network. The free parameters of the calibration standards are estimated by minimizing a figure of merit based on the expected equality of the S-parameters of the network when used in direct and reverse modes. The capabilities of the method are demonstrated through simulations, and real measurements are used to estimate the actual offset delay of a 50-Ω calibration load that is assigned zero delay by the manufacturer. The estimated delay is 38.8 ps with a 1 σ uncertainty of 2.1 ps for this particular load. This result is verified through measurements of a terminated airline. The measurements agree better with theoretical models of the airline when the reference plane is calibrated using the new estimate for the load delay.
    Keywords: Standards ; Transmission Line Measurements ; Calibration ; Delays ; Scattering Parameters ; Ports (Computers) ; Extraterrestrial Measurements ; Delay ; Impedance ; Reflection Standards ; Scattering Parameters ; Vector Network Analyzer (Vna) ; Engineering
    ISSN: 0018-9480
    E-ISSN: 1557-9670
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  • 10
    In: Monthly Notices of the Royal Astronomical Society, 2012, Vol. 419(2), pp.1070-1084
    Description: 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.
    Keywords: Methods: Statistical ; Cosmology: Theory ; Diffuse Radiation ; Radio Lines: General
    ISSN: 0035-8711
    E-ISSN: 1365-2966
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