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Berlin Brandenburg

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  • 1
    Language: English
    In: The Astrophysical Journal, 2017, Vol.835(1), p.49 (13pp)
    Description: The EDGES High-Band experiment aims to detect the sky-average brightness temperature of the 21 cm signal from the epoch of reionization in the redshift range . To probe this redshifted signal, EDGES High-Band conducts single-antenna measurements in the frequency range 90–190 MHz from the Murchison Radio-astronomy Observatory in Western Australia. In this paper, we describe the current strategy for calibration of the EDGES High-Band receiver and report calibration results for the instrument used in the 2015–2016 observational campaign. We propagate uncertainties in the receiver calibration measurements to the antenna temperature using a Monte Carlo approach. We define a performance objective of 1 mK residual rms after modeling foreground subtraction from a fiducial temperature spectrum using a five-term polynomial. Most of the calibration uncertainties yield residuals of 1 mK or less at confidence. However, current uncertainties in the antenna and receiver reflection coefficients can lead to residuals of up to 20 mK even in low-foreground sky regions. These dominant residuals could be reduced by (1) improving the accuracy in reflection measurements, especially their phase, (2) improving the impedance match at the antenna-receiver interface, and (3) decreasing the changes with frequency of the antenna reflection phase.
    Keywords: Astrophysics - Instrumentation And Methods For Astrophysics ; Astrophysics - Cosmology And Nongalactic Astrophysics;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 2
    Language: English
    In: The Astrophysical Journal, 2018, Vol.863(1), p.11 (11pp)
    Description: We use the sky-average spectrum measured by EDGES High-band (90–190 MHz) to constrain parameters of early galaxies independent of the absorption feature at 78 MHz reported by Bowman et al. These parameters represent traditional models of cosmic dawn and the epoch of reionization produced with the 21cmFAST simulation code. The parameters considered are (1) the UV ionizing efficiency ( ζ ); (2) minimum halo virial temperature hosting efficient star-forming galaxies ( ); (3) integrated soft-band X-ray luminosity ( ); and (4) minimum X-ray energy escaping the first galaxies ( E 0 ), corresponding to a typical H i column density for attenuation through the interstellar medium. The High-band spectrum disfavors high values of and ζ , which correspond to signals with late absorption troughs and sharp reionization transitions. It also disfavors intermediate values of , which produce relatively deep and narrow troughs within the band. Specifically, we rule out (95% C.L.). We then combine the EDGES High-band data with constraints on the electron-scattering optical depth from Planck and the hydrogen neutral fraction from high- z quasars. This produces a lower degeneracy between ζ and than that reported by Greig & Mesinger using the Planck and quasar constraints alone. Our main result in this combined analysis is the estimate (95% C.L.). We leave the evaluation of 21 cm models using simultaneously data from EDGES Low- and High-band for future work.
    Keywords: Astrophysics - Cosmology And Nongalactic Astrophysics ; Astrophysics - Astrophysics Of Galaxies;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 3
    Language: English
    In: Publications of the Astronomical Society of the Pacific, 2017, Vol.129(973), p.035002 (12pp)
    Description: Multiple instruments are pursuing constraints on dark energy, observing reionization and opening a window on the dark ages through the detection and characterization of the 21 cm hydrogen line for redshifts ranging from ∼1 to 25. These instruments, including CHIME in the sub-meter and HERA in the meter bands, are wide-field arrays with multiple-degree beams, typically operating in transit mode. Accurate knowledge of their primary beams is critical for separation of bright foregrounds from the desired cosmological signals, but difficult to achieve through astronomical observations alone. Previous beam calibration work at low frequencies has focused on model verification and does not address the need of 21 cm experiments for routine beam mapping, to the horizon, of the as-built array. We describe the design and methodology of a drone-mounted calibrator, the External Calibrator for Hydrogen Observatories (ECHO), that aims to address this need. We report on a first set of trials to calibrate low-frequency dipoles at 137 MHz and compare ECHO measurements to an established beam-mapping system based on transmissions from the Orbcomm satellite constellation. We create beam maps of two dipoles at a 9° resolution and find sample noise ranging from 1% at the zenith to 100% in the far sidelobes. Assuming this sample noise represents the error in the measurement, the higher end of this range is not yet consistent with the desired requirement but is an improvement on Orbcomm. The overall performance of ECHO suggests that the desired precision and angular coverage is achievable in practice with modest improvements. We identify the main sources of systematic error and uncertainty in our measurements and describe the steps needed to overcome them.
    Keywords: Acoustic Waves ; Hydrogen ; Noise Pollution ; Hydrogen Lines ; Stars, Universe (524);
    ISSN: 00046280
    E-ISSN: 15383873
    Source: IOPscience (IOP Publishing)
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  • 4
    Language: English
    In: The Astronomical Journal, 2008, Vol.136(2), pp.641-648
    Description: The mean absolute brightness temperature of the diffuse radio background was measured as a function of frequency in a continuous band between 100 and 200 MHz over an effective solid angle of sr at high Galactic latitude. A spectral brightness temperature index of = 2.5 ± 0.1 ( S = 0.5) was derived from the observations, where the error limits are 3 and include estimates of the instrumental systematics. Zenith drift scans with central declination = 26.5° and spanning right ascensions 0 〈 〈 10 h yielded little variation in the mean spectral index. The mean absolute brightness temperature at = 150 MHz was found to reach a minimum of T = 237 ± 10 K at = 2.5 h. Combining these measurements with those of Haslam et al. yields a spectral index of = 2.52 ± 0.04 between 150 〈 〈 408 MHz.
    Keywords: Astrophysics;
    ISSN: 0004-6256
    E-ISSN: 1538-3881
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  • 5
    Language: English
    In: The Astrophysical Journal, 2014, Vol.782(2), p.66 (25pp)
    Description: A number of experiments are currently working toward a measurement of the 21 cm signal from the epoch of reionization (EoR). Whether or not these experiments deliver a detection of cosmological emission, their limited sensitivity will prevent them from providing detailed information about the astrophysics of reionization. In this work, we consider what types of measurements will be enabled by the next generation of larger 21 cm EoR telescopes. To calculate the type of constraints that will be possible with such arrays, we use simple models for the instrument, foreground emission, and the reionization history. We focus primarily on an instrument modeled after the 0.1 km 2 collecting area Hydrogen Epoch of Reionization Array concept design and parameterize the uncertainties with regard to foreground emission by considering different limits to the recently described wedge footprint in k space. Uncertainties in the reionization history are accounted for using a series of simulations that vary the ionizing efficiency and minimum virial temperature of the galaxies responsible for reionization, as well as the mean free path of ionizing photons through the intergalactic medium. Given various combinations of models, we consider the significance of the possible power spectrum detections, the ability to trace the power spectrum evolution versus redshift, the detectability of salient power spectrum features, and the achievable level of quantitative constraints on astrophysical parameters. Ultimately, we find that 0.1 km 2 of collecting area is enough to ensure a very high significance ( 30) detection of the reionization power spectrum in even the most pessimistic scenarios. This sensitivity should allow for meaningful constraints on the reionization history and astrophysical parameters, especially if foreground subtraction techniques can be improved and successfully implemented.
    Keywords: Astrophysics - Cosmology And Nongalactic Astrophysics;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 6
    Language: English
    In: The Astrophysical Journal, 2009, Vol.695(1), pp.183-199
    Description: Subtraction of astrophysical foreground contamination from dirty sky maps produced by simulated measurements of the Murchison Widefield Array (MWA) has been performed by fitting a third-order polynomial along the spectral dimension of each pixel in the data cubes. The simulations are the first to include the unavoidable instrumental effects of the frequency-dependent primary antenna beams and synthesized array beams. They recover the one-dimensional spherically binned input redshifted 21 cm power spectrum within 1% over the scales probed most sensitively by the MWA (0.01 k 1 Mpc 1 ) and demonstrate that realistic instrumental effects will not mask the epoch of reionization signal. We find that the weighting function used to produce the dirty sky maps from the gridded visibility measurements is important to the success of the technique. Uniform weighting of the visibility measurements produces the best results, whereas natural weighting significantly worsens the foreground subtraction by coupling structure in the density of the visibility measurements to spectral structure in the dirty sky map data cube. The extremely dense uv -coverage of the MWA was found to be advantageous for this technique and produced very good results on scales corresponding to | u | 500 in the uv -plane without any selective editing of the uv -coverage.
    Keywords: Astrophysics;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 7
    Language: English
    In: The Astrophysical Journal, 2006, Vol.648(2), pp.767-773
    Description: Statistical observations of the epoch of reionization using the 21 cm line of neutral hydrogen have the potential to revolutionize our understanding of structure formation and the first luminous objects. However, these observations are complicated by a host of strong foreground sources. Several foreground-removal techniques have been proposed in the literature, and it has been assumed that these would be used in combination to reveal the epoch of reionization (EOR) signal. By studying the characteristic subtraction errors of the proposed foreground-removal techniques, we identify an additional subtraction stage that can further reduce the EOR foreground contamination, and study the interactions between the foreground-removal algorithms. This enables us to outline a comprehensive foreground-removal strategy that incorporates all previously proposed subtraction techniques. Using this foreground-removal framework and the characteristic subtraction errors, we discuss the complementarity of different foreground-removal techniques and the implications for array design and the analysis of EOR data.
    Keywords: Astrophysics;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 8
    In: 2012 ApJ 759 17
    Description: We introduce the Fast Holographic Deconvolution method for analyzing interferometric radio data. Our new method is an extension of A-projection/software-holography/forward modeling analysis techniques and shares their precision deconvolution and widefield polarimetry, while being significantly faster than current implementations that use full direction-dependent antenna gains. Using data from the MWA 32 antenna prototype, we demonstrate the effectiveness and precision of our new algorithm. Fast Holographic Deconvolution may be particularly important for upcoming 21 cm cosmology observations of the Epoch of Reionization and Dark Energy where foreground subtraction is intimately related to the precision of the data reduction. Comment: ApJ accepted
    Keywords: Astrophysics - Instrumentation And Methods For Astrophysics ; Astrophysics - Cosmology And Extragalactic Astrophysics
    ISSN: 0004637X
    E-ISSN: 15384357
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  • 9
    Language: English
    In: The Astrophysical Journal, 2014, Vol.793(1), p.28 (13pp)
    Description: 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 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, which has a wide primary beam. We estimate the EoR power based on a cross-correlation of visibilities over time and show that the drift scan strategy is capable of detecting the EoR signal with a signal to noise that is comparable/better 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.
    Keywords: Astrophysics - Instrumentation And Methods For Astrophysics ; Astrophysics - Cosmology And Nongalactic Astrophysics;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 10
    Description: 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. Comment: 15 pages, 14 figures, Accepted for publication in ApJ
    Keywords: Astrophysics - Solar And Stellar Astrophysics
    ISSN: 0004637X
    E-ISSN: 15384357
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