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Berlin Brandenburg

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  • 1
    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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  • 2
    Description: We report absolutely calibrated measurements of diffuse radio emission between 90 and 190 MHz from the Experiment to Detect the Global EoR Signature (EDGES). 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 211 nights of data acquired from July 2015 to March 2016. We derive the spectral index, $\beta$, as a function of local sidereal time (LST) and find -2.60 〉 $\beta$ 〉 -2.62 $\pm$0.02 between 0 and 12 h LST. When the Galactic Centre is in the sky, the spectral index flattens, reaching $\beta$ = -2.50 $\pm$0.02 at 17.7 h. The EDGES instrument is shown to be very stable throughout the observations with night-to-night reproducibility of $\sigma_{\beta}$ 〈 0.003. Including systematic uncertainty, the overall uncertainty of $\beta$ is 0.02 across all LST bins. These results improve on the earlier findings of Rogers & Bowman (2008) by reducing the spectral index uncertainty from 0.10 to 0.02 while considering more extensive sources of errors. We compare our measurements with spectral index simulations derived from the Global Sky Model (GSM) of de Oliveira-Costa et al. (2008) and with fits between the Guzm\'an et al. (2011) 45 MHz and Haslam et al. (1982) 408 MHz maps. We find good agreement at the transit of the Galactic Centre. Away from transit, the GSM tends to over-predict (GSM less negative) by 0.05 〈 $\Delta_{\beta} = \beta_{\text{GSM}}-\beta_{\text{EDGES}}$ 〈 0.12, while the 45-408 MHz fits tend to over-predict by $\Delta_{\beta}$ 〈 0.05.
    Keywords: Astrophysics - Instrumentation And Methods For Astrophysics ; Astrophysics - Astrophysics Of Galaxies
    ISSN: 00358711
    E-ISSN: 13652966
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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
    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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  • 5
    Language: English
    In: The Astrophysical Journal, 2004, Vol.617(1), pp.81-101
    Description: Observational measurements of the relationship between supermassive black holes (SMBHs) and the properties of their host galaxies are an important method for probing theoretical hierarchical growth models. Gravitational lensing is a unique mechanism for acquiring this information in systems at cosmologically significant redshifts. We review the calculations required to include SMBHs in two standard galactic lens models, a cored isothermal sphere and a broken power law. The presence of the SMBH produces two primary effects depending on the lens configuration, either blocking the "core" image that is usually predicted to form from a softened lens model or adding an extra, highly demagnified image to the predictions of the unaltered lens model. The magnitudes of these effects are very sensitive to galaxy core sizes and SMBH masses. Therefore, observations of these lenses would probe the properties of the inner regions of galaxies, including their SMBHs. Lensing cross sections and optical depth calculations indicate that to fully observe these characteristic signatures, flux ratios of order 10 6 or more between the brightest and faintest images of the lens must be detectable, and thus, the next generation of radio telescope technology offers the first opportunity for a serious observational campaign. Core images, however, are already detectable, and with additional observations their statistics may be used to guide future SMBH searches.
    Keywords: Astrophysics;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 6
    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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  • 7
    In: Monthly Notices of the Royal Astronomical Society, 2015, Vol. 447(3), pp.2468-2478
    Description: Recent observations with the Murchison Widefield Array at 185 MHz have serendipitously unveiled a heretofore unknown giant and relatively nearby ( z  = 0.0178) radio galaxy associated with NGC 1534. The diffuse emission presented here is the first indication that NGC 1534 is one of a rare class of objects (along with NGC 5128 and NGC 612) in which a galaxy with a prominent dust lane hosts radio emission on scales of ∼700 kpc. We present details of the radio emission along with a detailed comparison with other radio galaxies with discs. NGC 1534 is the lowest surface brightness radio galaxy known with an estimated scaled 1.4-GHz surface brightness of just 0.2 mJy arcmin −2 . The radio lobes have one of the steepest spectral indices yet observed: α = −2.1 ± 0.1, and the core to lobe luminosity ratio is 〈0.1 per cent. We estimate the space density of this low brightness (dying) phase of radio galaxy evolution as 7 × 10 −7  Mpc −3 and argue that normal AGN cannot spend more than 6 per cent of their lifetime in this phase if they all go through the same cycle.
    Keywords: Techniques: Interferometric ; Galaxies: Active ; Galaxies: General ; Galaxies: Individual:Ngc 1534 ; Radio Continuum: Galaxies
    ISSN: 0035-8711
    E-ISSN: 1365-2966
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  • 8
    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
    Description: Using the Murchison Widefield Array (MWA), the low-frequency Square Kilometre Array (SKA1 LOW) precursor located in Western Australia, we have completed the GaLactic and Extragalactic All-sky MWA (GLEAM) survey, and present the resulting extragalactic catalogue, utilising the first year of observations. The catalogue covers 24,831 square degrees, over declinations south of $+30^\circ$ and Galactic latitudes outside $10^\circ$ of the Galactic plane, excluding some areas such as the Magellanic Clouds. It contains 307,455 radio sources with 20 separate flux density measurements across 72--231MHz, selected from a time- and frequency- integrated image centred at 200MHz, with a resolution of $\approx 2$'. Over the catalogued region, we estimate that the catalogue is 90% complete at 170mJy, and 50% complete at 55mJy, and large areas are complete at even lower flux density levels. Its reliability is 99.97% above the detection threshold of $5\sigma$, which itself is typically 50mJy. These observations constitute the widest fractional bandwidth and largest sky area survey at radio frequencies to date, and calibrate the low frequency flux density scale of the southern sky to better than 10%. This paper presents details of the flagging, imaging, mosaicking, and source extraction/characterisation, as well as estimates of the completeness and reliability. All source measurements and images are available online (http://www.mwatelescope.org/science/gleam-survey). This is the first in a series of publications describing the GLEAM survey results. Comment: 30 pages, 18 figures, 6 tables, published in Monthly Notices of the Royal Astronomical Society
    Keywords: Astrophysics - Astrophysics Of Galaxies
    ISSN: 00358711
    E-ISSN: 13652966
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  • 10
    Language: English
    In: Publications of the Astronomical Society of the Pacific, 2017, Vol.129(974), p.045001 (27pp)
    Description: The Hydrogen Epoch of Reionization Array (HERA) is a staged experiment to measure 21 cm emission from the primordial intergalactic medium (IGM) throughout cosmic reionization ( z  = 6–12), and to explore earlier epochs of our Cosmic Dawn ( z  ∼ 30). During these epochs, early stars and black holes heated and ionized the IGM, introducing fluctuations in 21 cm emission. HERA is designed to characterize the evolution of the 21 cm power spectrum to constrain the timing and morphology of reionization, the properties of the first galaxies, the evolution of large-scale structure, and the early sources of heating. The full HERA instrument will be a 350-element interferometer in South Africa consisting of 14 m parabolic dishes observing from 50 to 250 MHz. Currently, 19 dishes have been deployed on site and the next 18 are under construction. HERA has been designated as an SKA Precursor instrument. In this paper, we summarize HERA’s scientific context and provide forecasts for its key science results. After reviewing the current state of the art in foreground mitigation, we use the delay-spectrum technique to motivate high-level performance requirements for the HERA instrument. Next, we present the HERA instrument design, along with the subsystem specifications that ensure that HERA meets its performance requirements. Finally, we summarize the schedule and status of the project. We conclude by suggesting that, given the realities of foreground contamination, current-generation 21 cm instruments are approaching their sensitivity limits. HERA is designed to bring both the sensitivity and the precision to deliver its primary science on the basis of proven foreground filtering techniques, while developing new subtraction techniques to unlock new capabilities. The result will be a major step toward realizing the widely recognized scientific potential of 21 cm cosmology.
    Keywords: Astrophysics - Instrumentation And Methods For Astrophysics ; Astrophysics - Cosmology And Nongalactic Astrophysics;
    ISSN: 00046280
    E-ISSN: 15383873
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