117 research outputs found
On the absence of gravitational lensing of the cosmic microwave background
The magnification of distant sources by mass clumps at lower ()
redshifts is calculated analytically. The clumps are initially assumed to be
galaxy group isothermal spheres with properties inferred from an extensive
survey. The average effect, which includes strong lensing, is exactly
counteracted by the beam divergence in between clumps (more precisely, the
average reciprocal magnification cancels the inverse Dyer-Roeder
demagnification). This conclusion is in fact independent of the matter density
function within each clump, and remains valid for arbitrary densities of matter
and dark energy. When tested against the CMB, a rather large lensing induced
{\it dispersion} in the angular size of the primary acoustic peaks of the TT
power spectrum is inconsistent with WMAP observations. The situation is
unchanged by the use of NFW profiles for the density distribution of groups.
Finally, our formulae are applied to an ensemble of NFW mass clumps or
isothermal spheres having the parameters of galaxy {\it clusters}. The acoustic
peak size dispersion remains unobservably large, and is also excluded by WMAP.
For galaxy groups, two possible ways of reconciling with the data are proposed,
both exploiting maximally the uncertainties in our knowledge of group
properties. The same escape routes are not available in the case of clusters,
however, because their properties are well understood. Here we have a more
robust conclusion: neither of the widely accepted models are good description
of clusters, or important elements of physics responsible for shaping zero
curvature space are missing from the standard cosmological model. When all the
effects are accrued, it is difficult to understand how WMAP could reveal no
evidence whatsoever of lensing by groups and clusters.Comment: ApJ v628, pp. 583-593 (August 1, 2005
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Harmonization of space-borne infra-red sensors measuring sea surface temperature
Sea surface temperature (SST) is observed by a constellation of sensors, and SST retrievals
are commonly combined into gridded SST analyses and climate data records (CDRs). Differential
biases between SSTs from different sensors cause errors in such products, including feature artefacts.
We introduce a new method for reducing differential biases across the SST constellation, by reconciling
the brightness temperature (BT) calibration and SST retrieval parameters between sensors. We use the
Advanced Along-Track Scanning Radiometer (AATSR) and the Sea and Land Surface Temperature
Radiometer (SLSTR) as reference sensors, and the Advanced Very High Resolution Radiometer
(AVHRR) of the MetOp-A mission to bridge the gap between these references. Observations across a
range of AVHRR zenith angles are matched with dual-view three-channel skin SST retrievals from
the AATSR and SLSTR. These skin SSTs act as the harmonization reference for AVHRR retrievals
by optimal estimation (OE). Parameters for the harmonized AVHRR OE are iteratively determined,
including BT bias corrections and observation error covariance matrices as functions of water-vapor
path. The OE SSTs obtained from AVHRR are shown to be closely consistent with the reference sensor
SSTs. Independent validation against drifting buoy SSTs shows that the AVHRR OE retrieval is stable
across the reference-sensor gap. We discuss that this method is suitable to improve consistency across
the whole constellation of SST sensors. The approach will help stabilize and reduce errors in future
SST CDRs, as well as having application to other domains of remote sensing
A survey of hard spectrum ROSAT sources 2: optical identification of hard sources
We have surveyed 188 ROSAT PSPC fields for X-ray sources with hard spectra
(alpha<0.5); such sources must be major contributors to the X-ray background at
faint fluxes. In this paper we present optical identifications for 62 of these
sources: 28 AGN which show broad lines in their optical spectra (BLAGN), 13
narrow emission line galaxies (NELGs), 5 galaxies with no visible emission
lines, 8 clusters and 8 Galactic stars. The BLAGN, NELGs and galaxies have
similar distributions of X-ray flux and spectra. Their ROSAT spectra are
consistent with their being AGN obscured by columns of 20.5<log(NH/cm^2)<23.
The hard spectrum BLAGN have a distribution of X-ray to optical ratios which is
similar to that found for AGN from soft X-ray surveys (1<alpha_OX<2). However,
a relatively large proportion (15%) of the BLAGN, NELGs and galaxies are radio
loud. This could be because the radio jets in these objects produce
intrinsically hard X-ray emission, or if their hardness is due to absorption,
it could be because radio loud objects are more X-ray luminous than radio quiet
objects. The 8 hard sources identified as clusters of galaxies are the
brightest, and softest group of sources and hence clusters are unlikely to be
an important component of the hard, faint population. We propose that BLAGN are
likely to constitute a significant fraction of the faint, hard, 0.5 - 2 keV
population and could be important to reproducing the shape of the X-ray
background, because they are the most numerous type of object in our sample
(comprising almost half the identified sources), and because all our high
redshift (z>1) identified hard sources have broad lines.Comment: 10 pages, 6 figures, accepted for publication in MNRA
A survey of hard spectrum ROSAT sources 1: X-ray source catalogue
We present a catalogue of 147 serendipitous X-ray sources selected to have
hard spectra (alpha < 0.5) from a survey of 188 ROSAT fields. Such sources must
be the dominant contributors to the X-ray background at faint fluxes. We have
used Monte Carlo simulations to verify that our technique is very efficient at
selecting hard sources: the survey has > 10 times as much effective area to
hard sources as it has to soft sources above a 0.5 - 2 keV flux level of 10^-14
erg/cm^2/s. The distribution of best fit spectral slopes of the hard sources
suggests that a typical ROSAT hard source in our survey has a spectral slope
alpha ~0. The hard sources have a steep number flux relation (dN/dS propto
S^-gamma with a best fit value of gamma = 2.72 +- 0.12) and make up about 15%
of all 0.5 - 2 keV sources with S > 10^-14 erg/cm^2/s. If their N(S) continues
to fainter fluxes, the hard sources will comprise ~ 40% of sources with 5
10^-15 < S < 10^-14. The population of hard sources can therefore account for
the harder average spectra of ROSAT sources with S < 10^-14. They probably make
a strong contribution to the X-ray background at faint fluxes and could be the
solution to the X-ray background spectral paradox.Comment: 13 pages, 6 figures, Accepted for publication in MNRA
XMM-Newton spectra of hard spectrum Rosat AGN: X-ray absorption and optical reddening
We present the XMM-Newton spectra of three low-redshift intermediate Seyferts
(one Sy1.5, and two Sy1.8), from our survey of hard spectrum Rosat sources. The
three AGN are well fitted by absorbed powerlaws, with intrinsic nuclear
photoelectric absorption from column densities between 1.3 and 4.0e21 cm-2. In
the brightest object the X-ray spectrum is good enough to show that the
absorber is not significantly ionized. For all three objects the powerlaw
slopes appear to be somewhat flatter (Gamma~1.3-1.6) than those found in
typical unabsorbed Seyferts. The constraints from optical and X-ray emission
lines imply that all three objects are Compton-thin. For the two fainter
objects, the reddening deduced from the optical broad emission lines in one of
them, and the optical continuum in the other, are similar to those expected
from the X-ray absorption, if we assume a Galactic gas-to-dust ratio and
reddening curve. The broad line region Balmer decrement of our brightest object
is larger than expected from its X-ray absorption, which can be explained
either by an intrinsic Balmer decrement with standard gas-to-dust ratio, or by
a >Galactic gas-to-dust ratio. These >=Galactic ratios of extinction to
photoelectric absorption cannot extend to the high redshift, high luminosity,
broad line AGN in our sample, because they have column densities >1e22 cm-2,
and so their broad line regions would be totally obscured. This means that some
effect (e.g., luminosity dependence, or evolution) needs to be present in order
to explain the whole population of absorbed AGN.Comment: LaTeX, 10 pages and 7 figures, A&A accepte
Inter-calibration of HY-1B/COCTS thermal infrared channels with MetOp-A/IASI
The Chinese Ocean Color and Temperature Scanner (COCTS) on board the Haiyang-1B (HY-1B) satellite has two thermal infrared channels (9 and 10) centred near 11 μm and 12 μm respectively which are intended for sea surface temperature (SST) observations. In order to improve the accuracy of COCTS SSTs, the inter-calibration of COCTS thermal infrared radiance is carried out. The Infrared Atmospheric Sounding Interferometer (IASI) on board MetOp-A satellite is used as inter-calibration reference owing to its hyperspectral nature and high-quality measurements. The inter-calibration of HY-1B COCTS thermal infrared radiances with IASI is undertaken for data from the period 2009 to 2011 located in the northwest Pacific. Collocations of COCTS radiance with IASI are identified within a temporal window of 30 minutes, a spatial window of 0.12° and an atmospheric path tolerance of 3%. Matched IASI spectra are convolved with the COCTS spectral response functions, while COCTS pixels within the footprint of each IASI pixel are spatially averaged, thus creating matched IASI-COCTS radiance pairs that should agree well in the absence of satellite biases. The radiances of COCTS 11 and 12 μm channel are lower than IASI with relatively large biases, and a strong dependence of difference on radiance in the case of 11 μm channel. We use linear robust regression for different four detectors of COCTS separately to obtain the inter-calibration coefficients to correct the COCTS radiance. After correction, the mean values of COCTS 11 and 12 μm channel minus IASI radiance are -0.02 mW m-2 cm sr-1 and -0.01 mW m-2 cm sr-1 respectively, with corresponding standard deviations of 0.51 mW m-2 cm sr-1 and 0.57 mW m-2 cm sr-1. Striped noise is present in COCTS original radiance imagery associated with inconsistency between four detectors, and inter-calibration is shown to reduce, although not eliminate, the striping. The calibration accuracy of COCTS is improved after inter-calibration, that is potentially useful for improving COCTS SST accuracy in the future
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Radiance uncertainty characterisation to facilitate climate data record creation
The uncertainty in a climate data records (CDRs) derived from Earth observations in part derives from the propagated uncertainty in the radiance record (the fundamental climate data record, FCDR) from which the geophysical estimates in the CDR are derived. A common barrier to providing uncertainty-quantified CDRs is the inaccessibility to CDR creators of appropriate radiance uncertainty information in the FCDR. Here, we propose radiance uncertainty information designed directly to facilitate estimation of propagated uncertainty in derived CDRs at full resolution and in gridded products. Errors in Earth observations are typically highly structured and complex, and the uncertainty information we propose is of intermediate complexity, sufficient to capture the main variability in propagated uncertainty in a CDR, while avoiding unfeasible complexity or data volume. The uncertainty and error correlation characteristics of uncertainty are quantified for three classes of error with different propagation properties: independent, structured and common radiance errors. The meaning, mathematical derivations, practical evaluation and example applications of this set of uncertainty information are presented
X-ray Spectroscopy of the Cluster of Galaxies Abell 1795 with XMM-Newton
The initial results from XMM-Newton observations of the rich cluster of
galaxies Abell 1795 are presented. The spatially-resolved X-ray spectra taken
by the European Photon Imaging Cameras (EPIC) show a temperature drop at a
radius of kpc from the cluster center, indicating that the ICM is
cooling. Both the EPIC and the Reflection Grating Spectrometers (RGS) spectra
extracted from the cluster center can be described by an isothermal model with
a temperature of keV. The volume emission measure of any cool
component ( keV) is less than a few % of the hot component at the cluster
center. A strong OVIII Lyman-alpha line was detected with the RGS from the
cluster core. The O abundance and its ratio to Fe at the cluster center is
0.2--0.5 and 0.5--1.5 times the solar value, respectively.Comment: Accepted: A&A Letters, 2001, 6 page
The Baldwin Effect and Black Hole Accretion: A Spectral Principal Component Analysis of a Complete QSO Sample
A unique set of UV-optical spectrograms of 22 low redshift QSOs are
investigated using principal component analysis. We find three significant
principal components over the broad wavelength range from Ly\alpha to H\alpha.
They together account for about 78% of the sample intrinsic variance. We
present strong arguments that the first principal component represents the
Baldwin effect, relating equivalent widths to the luminosity (i.e. accretion
rate), but only emission-line cores are involved. The second component
represents continuum variations, probably dominated by intrinsic reddening. The
third principal component directly relates QSO UV properties to the optical
principal component 1 found by Boroson & Green (1992). It is the primary cause
of scatter in the Baldwin relationships. It is also directly related to broad
emission-line width and soft X-ray spectral index, and therefore probably
driven by Eddington accretion ratio. We demonstrate how Baldwin relationships
can be derived using our first principal component, virtually eliminating the
scatter caused by the third principal component. This rekindles the hope that
the Baldwin relationships can be used for cosmological study.Comment: 35 pages, 13 figures, AASTEX, accepted for publication in Ap
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