The birth of the first luminous sources and the ensuing epoch of reionization are best studied via the redshifted 21-cm emission line, the signature of the first two imprinting the last. In this ...work, we present a fully Bayesian method, hibayes, for extracting the faint, global (sky-averaged) 21-cm signal from the much brighter foreground emission. We show that a simplified (but plausible) Gaussian model of the 21-cm emission from the Cosmic Dawn epoch (15 ≲ z ≲ 30), parametrized by an amplitude
$A_{\rm H\,\small {I}}$
, a frequency peak
$\nu _{\rm H\,\small {I}}$
and a width
$\sigma _{\rm H\,\small {I}}$
, can be extracted even in the presence of a structured foreground frequency spectrum (parametrized as a seventh-order polynomial), provided sufficient signal-to-noise (400 h of observation with a single dipole). We apply our method to an early, 19-min-long observation from the Large aperture Experiment to detect the Dark Ages, constraining the 21-cm signal amplitude and width to be
$-890 \lt A_{\rm H\,\small {I}} \lt 0$
mK and
$\sigma _{\rm H\,\small {I}} \gt 6.5$
MHz (corresponding to Δz > 1.9 at redshift z ≃ 20) respectively at the 95-per cent confidence level in the range 13.2 < z < 27.4 (100 > ν > 50 MHz).
The LWA1 Radio Telescope Ellingson, S. W.; Taylor, G. B.; Craig, J. ...
IEEE transactions on antennas and propagation,
05/2013, Letnik:
61, Številka:
5
Journal Article
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LWA1 is a new radio telescope operating in the frequency range 10-88 MHz, located in central New Mexico. The telescope consists of 258 pairs of dipole-type antennas whose outputs are individually ...digitized and formed into beams. Simultaneously, signals from all dipoles can be recorded using one of the instrument's "all dipoles" modes, facilitating all-sky imaging. Notable features of the instrument include high intrinsic sensitivity (≈ 6 kJy zenith system equivalent flux density), large instantaneous bandwidth (up to 78 MHz), and four independently steerable beams utilizing digital "true time delay" beamforming. This paper summarizes the design of LWA1 and its performance as determined in commissioning experiments. We describe the method currently in use for array calibration, and report on measurements of sensitivity and beamwidth.
Transmissions from ground-based systems in C- and X-bands present a significant challenge to the use of these bands for passive microwave remote sensing from aircraft and satellites. Because future ...missions plan to continue to use these frequencies, it is important to characterize and understand the nature of interference in as much of the candidate spectrum as possible. This paper presents a statistical analysis of interference observed in the continental U.S. using six months of data collected from the C- and X-band channels of the WindSat microwave radiometer. Our findings are consistent with those of previous studies by Li et al. and Njoku et al., which are based on data obtained from the Advanced Microwave Scanning Radiometer-EOS using somewhat similar center frequencies and bandwidths. Results show significant radio-frequency interference (RFI) at C-band, including brightnesses in horizontal and vertical polarizations in excess of 330 K, while X-band RFI is less obvious through direct examination of measured linearly polarized brightnesses. Evidence of lower levels of RFI is provided through use of the spectral and polarization indexes of Li et al., which reveal likely RFI contributions at X-band as well. Further confirmation of X-band RFI is obtained through analysis of the polarimetric channels, which are shown to provide direct evidence of RFI in contrast to the linearly polarized channels. A temporal analysis of the largest C-band RFI sources is also provided in an attempt to further understand their properties.
The next generation of large telescopes for radio astronomy at frequencies below 100 MHz will consist of tens of thousands of wide-band dipole-like antennas, each individually instrumented with a ...receiver and combined using digital signal processing. At these frequencies, the sensitivity of a telescope is limited by Galactic noise, with the result that even simple dipoles can deliver extraordinary useable bandwidth. In this paper the necessary characteristics for these antennas are explained, some bounds on performance are developed, and a few candidate designs are analyzed. It is shown that antenna systems consisting of simple wire dipoles, a 360 K active balun, and a long coaxial feedline can achieve Galactic noise-limited performance over large portions of the range 10-100 MHz. It is further shown that when these antennas are used as elements in a compact array, their Galactic noise-limited characteristics are not significantly affected.
A number of new and planned radio telescopes will consist of large arrays of low-gain antennas operating at frequencies below 300 MHz. In this frequency regime, Galactic noise can be a significant or ...dominant contribution to the total noise. This, combined with mutual coupling between antennas, makes it difficult to predict the sensitivity of these instruments. This paper describes a system model and procedure for estimating the system equivalent flux density (SEFD)-a useful and meaningful metric of the sensitivity of a radio telescope-that accounts for these issues. The method is applied to LWA-1, the first "station" of the Long Wavelength Array (LWA) interferometer. LWA-1 consists of 512 bowtie-type antennas within a 110 × 100 m elliptical footprint, and is designed to operate between 10 MHz and 88 MHz using receivers having noise temperature of about 250 K. It is shown that the correlation of Galactic noise between antennas significantly desensitizes the array for beam pointings which are not close to the zenith. It is also shown that considerable improvement is possible using beamforming coefficients which are designed to optimize signal-to-noise ratio under these conditions. Mutual coupling is found to play a significant role, but does not have a consistently positive or negative influence. In particular, we demonstrate that pattern multiplication (assuming the behavior of single antennas embedded in the array is the same as those same antennas by themselves) does not generate reliable estimates of SEFD.
Mental health disorders commonly co-occur, even between conceptually distinct syndromes, such as internalizing and externalizing disorders. The current study investigated whether phenotypic, genetic, ...and environmental variance in negative emotionality and behavioral control account for the covariation between major depressive disorder (MDD) and alcohol use disorder (AUD).
A total of 3623 members of a national twin registry were administered structured diagnostic telephone interviews that included assessments of lifetime histories of MDD and AUD, and were mailed self-report personality questionnaires that assessed stress reactivity (SR) and behavioral control (CON). A series of biometric models were fitted to partition the proportion of covariance between MDD and AUD into SR and CON.
A statistically significant proportion of the correlation between MDD and AUD was due to variance specific to SR (men = 0.31, women = 0.27) and CON (men = 0.20, women = 0.19). Further, genetic factors explained a large proportion of this correlation (0.63), with unique environmental factors explaining the rest. SR explained a significant proportion of the genetic (0.33) and environmental (0.23) overlap between MDD and AUD. In contrast, variance specific to CON accounted for genetic overlap (0.32), but not environmental overlap (0.004). In total, SR and CON accounted for approximately 70% of the genetic and 20% of the environmental covariation between MDD and AUD.
This is the first study to demonstrate that negative emotionality and behavioral control confer risk for the co-occurrence of MDD and AUD via genetic factors. These findings are consistent with the aims of NIMH's RDoC proposal to elucidate how transdiagnostic risk factors drive psychopathology.
DETECTION OF RADIO EMISSION FROM FIREBALLS Obenberger, K S; Taylor, G B; Hartman, J M ...
Astrophysical journal. Letters,
06/2014, Letnik:
788, Številka:
2
Journal Article
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We present the findings from the Prototype All-Sky Imager, a back end correlator of the first station of the Long Wavelength Array, which has recorded over 11,000 hr of all-sky images at frequencies ...between 25 and 75 MHz. In a search of this data for radio transients, we have found 49 long-duration (10 s of seconds) transients. Ten of these transients correlate both spatially and temporally with large meteors (fireballs), and their signatures suggest that fireballs emit a previously undiscovered low frequency, non-thermal pulse. This emission provides a new probe into the physics of meteors and identifies a new form of naturally occurring radio transient foreground.
A reconfigurable intelligent surface (RIS) employs an array of individually-controllable elements to scatter incident signals in a desirable way; for example, to facilitate links between base ...stations and mobile stations that would otherwise be blocked. A principal consideration in the study of RIS-enabled propagation channels is path loss. This paper presents a simple yet broadly-applicable method for calculating the path loss of a channel consisting of a passive reflectarray-type RIS. This model is then used to characterize path loss as a function of RIS size, link geometry, and the method used to set the element states. Whereas previous work presumes either (1) an array of parameterizable element patterns and spacings (most useful for analysis of specific designs) or (2) a continuous electromagnetic surface (most useful for determining scaling laws and theoretical limits), this work begins with (1) and is then shown to be consistent with (2), making it possible to identify specific practical designs and scenarios that exhibit the performance predicted using (2). This model is used to further elucidate the matter of path loss of the RIS-enabled channel relative to that of the free space direct and specular reflection channels, which is an important consideration in the design of networks employing RIS technology.
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