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• 1
Article
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
Article
Description: We report the spectral index of diffuse radio emission between 50 and 100 MHz from data collected with two implementations of the Experiment to Detect the Global EoR Signature (EDGES) low-band system. EDGES employs a wide beam zenith-pointing dipole antenna centred on a declination of $-26.7^\circ$. We measure the sky brightness temperature as a function of frequency averaged over the EDGES beam from 244 nights of data acquired between 14 September 2016 to 27 August 2017. We derive the spectral index, $\beta$, as a function of local sidereal time (LST) using night-time data and a two-parameter fitting equation. We find $-2.59〈\beta〈-2.54 \pm 0.011$ between 0 and 12 h LST, ignoring ionospheric effects. When the Galactic Centre is in the sky, the spectral index flattens, reaching $\beta = -2.46 \pm 0.011$ at 18.2 h. The measurements are stable throughout the observations with night-to-night reproducibility of $\sigma_{\beta}〈0.004$ except for the LST range of 7 to 12 h. We compare our measurements with predictions from various global sky models and find that the closest match is with the spectral index derived from the Guzm{\'a}n and Haslam sky maps, similar to the results found with the EDGES high-band instrument for 90-190 MHz. Three-parameter fitting was also evaluated with the result that the spectral index becomes more negative by $\sim$0.02 and has a maximum total uncertainty of 0.016. We also find that the third parameter, the spectral index curvature, $\gamma$, is constrained to $-0.11〈\gamma〈-0.04$. Correcting for expected levels of night-time ionospheric absorption causes $\beta$ to become more negative by $0.008$ - $0.016$ depending on LST.
Keywords: Astrophysics - Instrumentation And Methods For Astrophysics ; Astrophysics - Astrophysics Of Galaxies
ISSN: 00358711
E-ISSN: 13652966
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• 3
Article
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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• 4
Article
Description: Above redshift 6, the dominant source of neutral hydrogen in the Universe shifts from localized clumps in and around galaxies and filaments to a pervasive, diffuse component of the intergalactic medium (IGM). This transition tracks the global neutral fraction of hydrogen in the IGM and can be studied, in principle, through the redshifted 21 cm hyperfine transition line. During the last half of the reionization epoch, the mean (global) brightness temperature of the redshifted 21 cm emission is proportional to the neutral fraction, but at earlier times (10 〈 z 〈 25), the mean brightness temperature should probe the spin temperature of neutral hydrogen in the IGM. Measuring the (of order 10 mK) mean brightness temperature of the redshifted 21 cm line as a function of frequency (and hence redshift) would chart the early evolution of galaxies through the heating and ionizing of the IGM by their stellar populations. Experiments are already underway to accomplish this task or, at least, provide basic constraints on the evolution of the mean brightness temperature. We provide a brief overview of one of these projects, the Experiment to the Detect the Global EOR Signature (EDGES), and discuss prospects for future results. Comment: From AIP Conference Proceedings, Volume 1035, 2008, "The Evolution of Galaxies through the Neutral Hydrogen Window". 3 pages
Keywords: Astrophysics - Galaxy Astrophysics
Source: Cornell University
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• 5
Conference Proceeding
Language: English
In: The Evolution Of Galaxies Through The Neutral Hydrogen Window, Arecibo Observatory (Puerto Rico) (1–3 February 2008):
In: AIP Conference Proceedings, 01 August 2008, Vol.1035(1), pp.87-89
Description: Above redshift 6, the dominant source of neutral hydrogen in the Universe shifts from localized clumps in and around galaxies and filaments to a pervasive, diffuse component of the intergalactic medium (IGM). This transition tracks the global neutral fraction of hydrogen in the IGM and can be studied, in principle, through the redshifted 21 cm hyperfine transition line. During the last half of the reionization epoch, the mean (global) brightness temperature of the redshifted 21 cm emission is proportional to the neutral fraction, but at earlier times (10〈z〈25), the mean brightness temperature should probe the spin temperature of neutral hydrogen in the IGM. Measuring the (of order 10 mK) mean brightness temperature of the redshifted 21 cm line as a function of frequency (and hence redshift) would chart the early evolution of galaxies through the heating and ionizing of the IGM by their stellar populations. Experiments are already underway to accomplish this task or, at least, provide basic constraints on the evolution of the mean brightness temperature. We provide a brief overview of one of these projects, the Experiment to the Detect the Global EOR Signature (EDGES), and discuss prospects for future results.
Keywords: Astronomy and Astrophysics
ISBN: 978-0-7354-0558-5
ISSN: 0094-243X
E-ISSN: 1551-7616
Source: © 2008 American Institute of Physics (AIP)〈img src=http://exlibris-pub.s3.amazonaws.com/AIP_edited.gif style="vertical-align:middle;margin-left:7px"〉
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• 6
Article
Language: English
In: The Astrophysical Journal, 2008, Vol.676(1), pp.1-9
Description: Preliminary results are presented from a simple, single-antenna experiment designed to measure the all-sky radio spectrum between 100 and 200 MHz. The system used an internal comparison-switching scheme to reduce nonsmooth instrumental contaminants in the measured spectrum to 75 mK. From the observations, we place an initial upper limit of 450 mK on the relative brightness temperature of the redshifted 21 cm contribution to the spectrum due to neutral hydrogen in the intergalactic medium (IGM) during the epoch of reionization, assuming a rapid transition to a fully ionized IGM at z = 8. With refinement, this technique should be able to distinguish between slow and fast reionization scenarios. To constrain the duration of reionization to z 〉 2, the systemic residuals in the measured spectrum must be reduced to 3 mK.
Keywords: Astrophysics;
ISSN: 0004-637X
E-ISSN: 1538-4357
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• 7
Article
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 conducted over two nights with significantly different levels of ionospheric activity. We use these data to assess the impact of systematic errors at low frequency, including the ionosphere and radio-frequency interference, on a power spectrum measurement. We find that after the 1–3 h of integration presented here, our measurements at the Murchison Radio Observatory are not limited by RFI, even within the FM band, and that the ionosphere does not appear to affect the level of power in the modes that we expect to be sensitive to cosmology. Power spectrum detections, inconsistent with noise, due to fine spectral structure imprinted on the foregrounds by reflections in the signal-chain, occupy the spatial Fourier modes where we would otherwise be most sensitive to the cosmological signal. We are able to reduce this contamination using calibration solutions derived from autocorrelations so that we achieve an sensitivity of 10 4  mK on comoving scales k ≲ 0.5  h  Mpc −1 . This represents the first upper limits on the 21 cm power spectrum fluctuations at redshifts 12 ≲ z ≲ 18 but is still limited by calibration systematics. While calibration improvements may allow us to further remove this contamination, our results emphasize that future experiments should consider carefully the existence of and their ability to calibrate out any spectral structure within the EoR window.
Keywords: Techniques: Interferometric ; Dark Ages, Reionization, First Stars ; Radio Lines: General ; X - Rays: Galaxies
ISSN: 0035-8711
E-ISSN: 1365-2966
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• 8
Article
Language: English
In: The Astrophysical Journal, 2016, Vol.818(2), p.139 (18pp)
Description: Detection of the cosmological neutral hydrogen signal from the Epoch of Reionization (EoR) and estimation of its basic physical parameters are principal scientific aims of many current low-frequency radio telescopes. Here we describe the Cosmological H i Power Spectrum Estimator (CHIPS), an algorithm developed and implemented with data from the Murchison Widefield Array, to compute the two-dimensional and spherically-averaged power spectrum of brightness temperature fluctuations. The principal motivations for CHIPS are the application of realistic instrumental and foreground models to form the optimal estimator, thereby maximizing the likelihood of unbiased signal estimation, and allowing a full covariant understanding of the outputs. CHIPS employs an inverse-covariance weighting of the data through the maximum likelihood estimator, thereby allowing use of the full parameter space for signal estimation (“foreground suppression”). We describe the motivation for the algorithm, implementation, application to real and simulated data, and early outputs. Upon application to a set of 3 hr of data, we set a 2 σ upper limit on the EoR dimensionless power at Mpc −1 of mK 2 in the redshift range z  = [6.2–6.6], consistent with previous estimates.
Keywords: Astrophysics - Instrumentation And Methods For Astrophysics ; Astrophysics - Cosmology And Nongalactic Astrophysics;
ISSN: 0004-637X
E-ISSN: 1538-4357
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• 9
Article
Description: The Murchison Widefield Array (MWA) is a dipole-based aperture array synthesis telescope designed to operate in the 80-300 MHz frequency range. It is capable of a wide range of science investigations, but is initially focused on three key science projects. These are detection and characterization of 3-dimensional brightness temperature fluctuations in the 21cm line of neutral hydrogen during the Epoch of Reionization (EoR) at redshifts from 6 to 10, solar imaging and remote sensing of the inner heliosphere via propagation effects on signals from distant background sources,and high-sensitivity exploration of the variable radio sky. The array design features 8192 dual-polarization broad-band active dipoles, arranged into 512 tiles comprising 16 dipoles each. The tiles are quasi-randomly distributed over an aperture 1.5km in diameter, with a small number of outliers extending to 3km. All tile-tile baselines are correlated in custom FPGA-based hardware, yielding a Nyquist-sampled instantaneous monochromatic uv coverage and unprecedented point spread function (PSF) quality. The correlated data are calibrated in real time using novel position-dependent self-calibration algorithms. The array is located in the Murchison region of outback Western Australia. This region is characterized by extremely low population density and a superbly radio-quiet environment,allowing full exploitation of the instrumental capabilities. Comment: 9 pages, 5 figures, 1 table. Accepted for publication in Proceedings of the IEEE
Keywords: Astrophysics - Instrumentation And Methods For Astrophysics ; Astrophysics - Cosmology And Nongalactic Astrophysics ; Astrophysics - Solar And Stellar Astrophysics
ISSN: 00189219
E-ISSN: 15582256
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• 10
Article
Language: English
In: The Astrophysical Journal, 2007, Vol.665(1), pp.618-627
Description: We report on the detection of giant pulses from the Crab Nebula pulsar at a frequency of 200 MHz using the field deployment system designed for the Mileura Widefield Array's Low Frequency Demonstrator (MWA-LFD). Our observations are among the first high-quality detections at such low frequencies. The measured pulse shapes are deconvolved for interstellar pulse broadening, yielding a pulse-broadening time of 670 ± 100 s, and the implied strength of scattering (scattering measure) is the lowest that is estimated toward the Crab Nebula from observations made so far. The sensitivity of the system is largely dictated by the sky background, and our simple equipment is capable of detecting pulses that are brighter than ~9 kJy in amplitude. The brightest giant pulse detected in our data has a peak amplitude of ~50 kJy, and the implied brightness temperature is 10 31.6 K. We discuss the giant pulse detection prospects with the full MWA-LFD system. With a sensitivity over 2 orders of magnitude larger than the prototype equipment, the full system will be capable of detecting such bright giant pulses out to a wide range of Galactic distances; from ~15 to ~30 kpc depending on the frequency. The MWA-LFD will thus be a highly promising instrument for the studies of giant pulses and other fast radio transients at low frequencies.
Keywords: Galaxies ; Pulsars ; Brightness Temperature ; Stars, Universe (524);
ISSN: 0004-637X
E-ISSN: 1538-4357
Source: IOPscience (IOP Publishing)
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