1,579 research outputs found
The MAP Satellite Feed Horns
We present the design, manufacturing methods, and characterization of 20
microwave feed horns currently in use on the Microwave Anisotropy Probe (MAP)
satellite. The nature of the cosmic microwave background (CMB) anisotropy
requires a detailed understanding of the properties of every optical component
of a microwave telescope. In particular, the properties of the feeds must be
known so that the forward gain and sidelobe response of the telescope can be
modeled and so that potential systematic effects may be computed. MAP requires
low emissivity, azimuthally symmetric, low-sidelobe feeds in five microwave
bands (K, Ka, Q, V, and W) that fit within a constrained geometry. The beam
pattern of each feed is modeled and compared with measurements; the agreement
is generally excellent to the -60 dB level (80 degrees from the beam peak).
This agreement verifies the beam-predicting software and the manufacturing
process. The feeds also affect the properties and modeling of the microwave
receivers. To this end, we show that the reflection from the feeds is less than
-25 dB over most of each band and that their emissivity is acceptable. The
feeds meet their multiple requirements.Comment: 9 pages with 7 figures, of which 2 are in low-resolution versions;
paper is available with higher quality figures at
http://map.gsfc.nasa.gov/m_mm/tp_links.htm
Expression and Function of GABA Receptors in Myelinating Cells
Myelin facilitates the fast transmission of nerve impulses and provides metabolic support to axons. Differentiation of oligodendrocyte progenitor cells (OPCs) and Schwann cell (SC) precursors is critical for myelination during development and myelin repair in demyelinating disorders. Myelination is tightly controlled by neuron-glia communication and requires the participation of a wide repertoire of signals, including neurotransmitters such as glutamate, ATP, adenosine, or gamma-aminobutyric acid (GABA). GABA is the main inhibitory neurotransmitter in the central nervous system (CNS) and it is also present in the peripheral nervous system (PNS). The composition and function of GABA receptors (GABARs) are well studied in neurons, while their nature and role in glial cells are still incipient. Recent studies demonstrate that GABA-mediated signaling mechanisms play relevant roles in OPC and SC precursor development and function, and stand out the implication of GABARs in oligodendrocyte (OL) and SC maturation and myelination. In this review, we highlight the evidence supporting the novel role of GABA with an emphasis on the molecular identity of the receptors expressed in these glial cells and the possible signaling pathways involved in their actions. GABAergic signaling in myelinating cells may have potential implications for developing novel reparative therapies in demyelinating diseases.This work was supported by CIBERNED (CB06/05/0076; CM) and by grants from the Ministry of Economy and Competitiveness, Government of Spain (SAF2016-75292-R and PID2019-109724RB-I00; CM), Basque Government (IT1203-19; CM), CONACYT-Mexico (No. 252121; RA), PAPIIT-UNAM-Mexico (IN203519; RA) and NIH (R21AG053740 and R21MH113177; AL). MS-R was hired thanks to the Gangoiti Foundation (Bilbao). LB-C and RO hold fellowships from Basque Government and CONACYT-Mexico, respectively
First Season QUIET Observations: Measurements of CMB Polarization Power Spectra at 43 GHz in the Multipole Range 25 <= ell <= 475
The Q/U Imaging ExperimenT (QUIET) employs coherent receivers at 43GHz and
95GHz, operating on the Chajnantor plateau in the Atacama Desert in Chile, to
measure the anisotropy in the polarization of the CMB. QUIET primarily targets
the B modes from primordial gravitational waves. The combination of these
frequencies gives sensitivity to foreground contributions from diffuse Galactic
synchrotron radiation. Between 2008 October and 2010 December, >10,000hours of
data were collected, first with the 19-element 43GHz array (3458hours) and then
with the 90-element 95GHz array. Each array observes the same four fields,
selected for low foregrounds, together covering ~1000deg^2. This paper reports
initial results from the 43GHz receiver which has an array sensitivity to CMB
fluctuations of 69uK sqrt(s). The data were extensively studied with a large
suite of null tests before the power spectra, determined with two independent
pipelines, were examined. Analysis choices, including data selection, were
modified until the null tests passed. Cross correlating maps with different
telescope pointings is used to eliminate a bias. This paper reports the EE, BB
and EB power spectra in the multipole range ell=25-475. With the exception of
the lowest multipole bin for one of the fields, where a polarized foreground,
consistent with Galactic synchrotron radiation, is detected with 3sigma
significance, the E-mode spectrum is consistent with the LCDM model, confirming
the only previous detection of the first acoustic peak. The B-mode spectrum is
consistent with zero, leading to a measurement of the tensor-to-scalar ratio of
r=0.35+1.06-0.87. The combination of a new time-stream double-demodulation
technique, Mizuguchi-Dragone optics, natural sky rotation, and frequent
boresight rotation leads to the lowest level of systematic contamination in the
B-mode power so far reported, below the level of r=0.1Comment: 19 pages, 14 figures, higher quality figures are available at
http://quiet.uchicago.edu/results/index.html; Fixed a typo and corrected
statistical error values used as a reference in Figure 14, showing our
systematic uncertainties (unchanged) vs. multipole; Revision to ApJ accepted
version, this paper should be cited as "QUIET Collaboration et al. (2011)
Cosmology of neutrinos and extra light particles after WMAP3
We study how present data probe standard and non-standard properties of
neutrinos and the possible existence of new light particles, freely-streaming
or interacting, among themselves or with neutrinos. Our results include: sum
m_nu < 0.40 eV at 99.9% C.L.; that extra massless particles have abundance
Delta N_nu = 2 pm 1 if freely-streaming and Delta N_nu = 0 pm 1.3 if
interacting; that 3 interacting neutrinos are disfavored at about 4 sigma. We
investigate the robustness of our results by fitting to different sub-sets of
data. We developed our own cosmological computational tools, somewhat different
from the standard ones.Comment: 18 pages, 8 figures. Added in v2: an explicit comparison of our code
with CAMB, some clarifications on the statistical analysis and some
references. Matches version published in JCA
The QUIET Instrument
The Q/U Imaging ExperimenT (QUIET) is designed to measure polarization in the
Cosmic Microwave Background, targeting the imprint of inflationary
gravitational waves at large angular scales (~ 1 degree). Between 2008 October
and 2010 December, two independent receiver arrays were deployed sequentially
on a 1.4 m side-fed Dragonian telescope. The polarimeters which form the focal
planes use a highly compact design based on High Electron Mobility Transistors
(HEMTs) that provides simultaneous measurements of the Stokes parameters Q, U,
and I in a single module. The 17-element Q-band polarimeter array, with a
central frequency of 43.1 GHz, has the best sensitivity (69 uK sqrt(s)) and the
lowest instrumental systematic errors ever achieved in this band, contributing
to the tensor-to-scalar ratio at r < 0.1. The 84-element W-band polarimeter
array has a sensitivity of 87 uK sqrt(s) at a central frequency of 94.5 GHz. It
has the lowest systematic errors to date, contributing at r < 0.01. The two
arrays together cover multipoles in the range l= 25-975. These are the largest
HEMT-based arrays deployed to date. This article describes the design,
calibration, performance of, and sources of systematic error for the
instrument
First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Data Processing Methods and Systematic Errors Limits
We describe the calibration and data processing methods used to generate
full-sky maps of the cosmic microwave background (CMB) from the first year of
Wilkinson Microwave Anisotropy Probe (WMAP) observations. Detailed limits on
residual systematic errors are assigned based largely on analyses of the flight
data supplemented, where necessary, with results from ground tests. The data
are calibrated in flight using the dipole modulation of the CMB due to the
observatory's motion around the Sun. This constitutes a full-beam calibration
source. An iterative algorithm simultaneously fits the time-ordered data to
obtain calibration parameters and pixelized sky map temperatures. The noise
properties are determined by analyzing the time-ordered data with this sky
signal estimate subtracted. Based on this, we apply a pre-whitening filter to
the time-ordered data to remove a low level of 1/f noise. We infer and correct
for a small ~1% transmission imbalance between the two sky inputs to each
differential radiometer, and we subtract a small sidelobe correction from the
23 GHz (K band) map prior to further analysis. No other systematic error
corrections are applied to the data. Calibration and baseline artifacts,
including the response to environmental perturbations, are negligible.
Systematic uncertainties are comparable to statistical uncertainties in the
characterization of the beam response. Both are accounted for in the covariance
matrix of the window function and are propagated to uncertainties in the final
power spectrum. We characterize the combined upper limits to residual
systematic uncertainties through the pixel covariance matrix.Comment: One of 13 companion papers on first-year WMAP results submitted to
ApJ; 58 pages with 14 figures; a version with higher quality figures is at
http://lambda.gsfc.nasa.gov/product/map/map_bibliography.htm
Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Beam Profiles, Data Processing, Radiometer Characterization and Systematic Error Limits
The WMAP satellite has completed 3 years of observations of the cosmic
microwave background radiation. The 3-year data products include several sets
of full sky maps of the Stokes I, Q and U parameters in 5 frequency bands,
spanning 23 to 94 GHz, and supporting items, such as beam window functions and
noise covariance matrices. The processing used to produce the current sky maps
and supporting products represents a significant advancement over the first
year analysis, and is described herein. Improvements to the pointing
reconstruction, radiometer gain modeling, window function determination and
radiometer spectral noise parametrization are presented. A detailed description
of the updated data processing that produces maximum likelihood sky map
estimates is presented, along with the methods used to produce reduced
resolution maps and corresponding noise covariance matrices. Finally two
methods used to evaluate the noise of the full resolution sky maps are
presented along with several representative year-to-year null tests,
demonstrating that sky maps produced from data from different observational
epochs are consistent.Comment: 58 pgs, 16 figs. Accepted version of the 3-year paper as posted on
http://lambda.gsfc.nasa.gov/product/map/dr2/map_bibliography.cfm in January
200
The Atacama Cosmology Telescope (ACT): Beam Profiles and First SZ Cluster Maps
The Atacama Cosmology Telescope (ACT) is currently observing the cosmic
microwave background with arcminute resolution at 148 GHz, 218 GHz, and 277
GHz. In this paper, we present ACT's first results. Data have been analyzed
using a maximum-likelihood map-making method which uses B-splines to model and
remove the atmospheric signal. It has been used to make high-precision beam
maps from which we determine the experiment's window functions. This beam
information directly impacts all subsequent analyses of the data. We also used
the method to map a sample of galaxy clusters via the Sunyaev-Zel'dovich (SZ)
effect, and show five clusters previously detected with X-ray or SZ
observations. We provide integrated Compton-y measurements for each cluster. Of
particular interest is our detection of the z = 0.44 component of A3128 and our
current non-detection of the low-redshift part, providing strong evidence that
the further cluster is more massive as suggested by X-ray measurements. This is
a compelling example of the redshift-independent mass selection of the SZ
effect.Comment: 16 pages, 10 figures. Accepted for publication in ApJS. See Marriage
et al. (arXiv:1010.1065) and Menanteau et al. (arXiv:1006.5126) for
additional cluster result
Supernova / Acceleration Probe: A Satellite Experiment to Study the Nature of the Dark Energy
The Supernova / Acceleration Probe (SNAP) is a proposed space-based
experiment designed to study the dark energy and alternative explanations of
the acceleration of the Universe's expansion by performing a series of
complementary systematics-controlled measurements. We describe a
self-consistent reference mission design for building a Type Ia supernova
Hubble diagram and for performing a wide-area weak gravitational lensing study.
A 2-m wide-field telescope feeds a focal plane consisting of a 0.7
square-degree imager tiled with equal areas of optical CCDs and near infrared
sensors, and a high-efficiency low-resolution integral field spectrograph. The
SNAP mission will obtain high-signal-to-noise calibrated light-curves and
spectra for several thousand supernovae at redshifts between z=0.1 and 1.7. A
wide-field survey covering one thousand square degrees resolves ~100 galaxies
per square arcminute. If we assume we live in a cosmological-constant-dominated
Universe, the matter density, dark energy density, and flatness of space can
all be measured with SNAP supernova and weak-lensing measurements to a
systematics-limited accuracy of 1%. For a flat universe, the
density-to-pressure ratio of dark energy can be similarly measured to 5% for
the present value w0 and ~0.1 for the time variation w'. The large survey area,
depth, spatial resolution, time-sampling, and nine-band optical to NIR
photometry will support additional independent and/or complementary dark-energy
measurement approaches as well as a broad range of auxiliary science programs.
(Abridged)Comment: 40 pages, 18 figures, submitted to PASP, http://snap.lbl.go
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