105 research outputs found
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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
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
X-ray Spectra of the RIXOS source sample
We present results of an extensive study of the X-ray spectral properties of
sources detected in the RIXOS survey, that is nearly complete down to a flux
limit of 3e-14 cgs (0.5-2 keV). We show that for X-ray surveys containing
sources with low count rate spectral slopes estimated using simple hardness
ratios in the ROSAT band can be biased. Instead we analyse three-colour X-ray
data using statistical techniques appropriate to the Poisson regime which
removes the effects of this bias. We have then applied this technique to the
RIXOS survey to study the spectral properties of the sample. For the AGN we
find an average energy index of 1.05+-0.05 with no evidence for spectral
evolution with redshift. Individual AGN are shown to have a range of properties
including soft X-ray excesses and intrinsic absorption. Narrow Emission Line
Galaxies also seem to fit to a power-law spectrum, which may indicate a
non-thermal origin for their X-ray emission. We infer that most of the clusters
in the sample have a bremsstrahlung temperature >3 keV, although some show
evidence for a cooling flow. The stars deviate strongly from a power-law model
but fit to a thermal model. Finally, we have analysed the whole RIXOS sample
containing 1762 sources. We find that the mean spectral slope of the sources
hardens at lower fluxes in agreement with results from other samples. However,
a study of the individual sources demonstrates that the hardening of the mean
is caused by the appearance of a population of very hard sources at the lowest
fluxes. This has implications for the nature of the soft X-ray background.Comment: 31,LaTeX file, 2 PS files with Table 2 and 22 PS figures. MNRAS in
pres
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