262 research outputs found
A whole sky study of quasars known population starting from the LQAC-2 compiled catalogue
No full textContext. Thanks to huge surveys, such as the Sloan Digital Sky Survey
(SDSS), the last decade has shown a dramatic increase in the number of known quasars. In
the second release of the general compiled catalogue Large Quasar Astrometric Catalogue
(LQAC), 187 504 objects are recorded.
Aims. From this catalogue, we carry out statistical studies dealing with
several topics: the astrometric accuracy of the quasars, their spatial location, the
distribution of the distance to the closest neighbour, the identification of binary
quasars, the completness of catalogues at a given magnitude and the estimation of the
number of quasars expected to be detected by the astrometric space mission
Gaia.
Methods. We analyse the astrometric improvements brought by the LQAC-2
in terms of equatorial coordinates off-sets. We plot the bi-dimensional spatial
distribution of the LQAC-2 quasars according to their equatorial, galactic, and ecliptic
coordinates, thus exploring the anisotropy of the distribution. We compare the observed
distribution of closest neighbours with the theoretical values based on a Poisson
distribution. Moreover, we perform a comparison between two catalogues, the SDSS and the
2dF inside a huge common field. By extrapolating to the whole sky we deduce the number of
quasars that will be detected by Gaia.
Results. We show how the equatorial, ecliptic, and galactic distribution
of recorded quasars is strongly affected by the galactic extinction as well as by the
deficiency of detections in the southern hemisphere. In homogenous zones covered by the
SDSS survey we identify a significant excess of closest neighbours at short angular
distances, with respect to the theoretical estimation, which is caused by the presence of
binary quasars. Moreover, we detail the incompletness of systematic survey catalogues at
any magnitude threshold, when comparing two huge surveys such as the SDSS and 2dF.
Following this study we deduce the number of quasars detected by Gaia
under a given magnitude threshold. For V = 20, this number should be at least 1 million
objects
Gaia data release 2: Catalogue validation
No full textContext. The second Gaia data release (DR2) contains very precise astrometric and photometric properties for more than one billion sources, astrophysical parameters for dozens of millions, radial velocities for millions, variability information for half a million stars from selected variability classes, and orbits for thousands of solar system objects. Aims. Before the catalogue was published, these data have undergone dedicated validation processes. The goal of this paper is to describe the validation results in terms of completeness, accuracy, and precision of the various Gaia DR2 data. Methods. The validation processes include a systematic analysis of the catalogue content to detect anomalies, either individual errors or statistical properties, using statistical analysis and comparisons to external data or to models. Results. Although the astrometric, photometric, and spectroscopic data are of unprecedented quality and quantity, it is shown that the data cannot be used without dedicated attention to the limitations described here, in the catalogue documentation and in accompanying papers. We place special emphasis on the caveats for the statistical use of the data in scientific exploitation. In particular, we discuss the quality filters and the consideration of the properties, systematics, and uncertainties from astrometry to astrophysical parameters, together with the various selection functions
Cross-identifications and astro-photometric comparisons of the ICRF with recent catalogs of quasars
No full textThe ICRF represents the most accurate astrometric catalog that
is the primary reference frame adopted by the IAU General Assembly at Kyoto
in 1997.
Despite its importance, an exhaustive study of both the sky coverage
and the physical properties of the extragalactic sources representing the ICRF
is lacking. The object of the present paper is to extract some
information concerning these sources and cross-identify them with the data gathered by
Véron-Cetty & Véron (2003, A&A, 412, 399), which is the largest compilation of various
catalogs of quasars. After this cross-identification of the sources, we carry out
a study of some general features of the ICRF concerning the visual
magnitudes, the redshifts, and the flux at radio wavelength
Sidereal orientation of the Earth and stability of the VLBI celestial reference frame
No full textThe consideration of time stability of extragalactic
radio sources observed by VLBI is shown to allow the realisation of
more consistent celestial reference frames.
The impact on the estimation of
precession and nutation components is investigated
over the time span 1984–2002.
The precession correction to the IAU 2000 value that is obtained when
excluding the unstable sources reaches 49 ± 5 μas/year,
to be compared to 12 ± 5 μas/year using the current
conventional celestial frame. The determination of the obliquity rate
is unaffected and remains at the level of
27 ± 2 μas/year. The observed correction to the 18.6-year
nutation amplitude using the current
conventional celestial frame ie sizeably corrupted by the
unstable sources. After accounting for this effect, the estimations
relative to both sets of reference radio sources confirm a
discrepancy with the IAU 2000 nutation model
with a total amplitude of 320 ± 100 μas for the observed
nutation in longitude, to be compared to the 80 μas discrepancy found by
Mathews et al. (2002, JGRB, 107, 1029). The discrepancy in obliquity
amounts to 50 ± 16 μas. The effect of source
instability is shown to have an impact on the
determination of universal time at the one microsecond level.
The high and medium frequency nutation terms (up to periods
of a few years) are impacted only in the early years of the program.
Chapter 7 concerning the observation of
the core and inner core free nutations is paralleled by a
twin paper (Dehant et al. 2005, A&A, 438, 1149) that proposes
a theoretical development for their atmospheric and
oceanic excitation.
Analysis strategy issues for the maintenance of the ICRF axes
No full textIn preparation for the evolution of the definition of
the VLBI-based International Celestial Reference Frame (ICRF),
various elements of the analysis strategy are
investigated, such as reference source selection and the impact
of the status of the terrestrial
reference frame in the data analysis model.
We conclude that including the determination of both the
celestial and the terrestrial reference frames in the analysis
does not affect the quality of the celestial reference frame.
The determination of precession and nutation
components is not affected by the status of the terrestrial
reference frame but it is affected by the selection of reference
radio sources.
Extending an earlier study,
three lists of reference sources based on progressively larger
tolerances are proposed.
They include 181, 225 and 247 objects, respectively.
The continuous wavelet transform, an analysis tool for NMR spectroscopy
No full textThe discrete wavelet transform has been used in NMR spectroscopy by several authors. We show here that the continuous wavelet transform (CWT) is also an efficient tool in that context. After reviewing briefly the analysis of spectral lines with the CWT, we discuss two applications specific to NMR, namely the removal of a large unwanted line and the rephasing of a signal perturbed by eddy currents. More details on the CWT are given in two appendixes. (C) 1997 Academic Press
LQAC-6: Sixth Release of the Large Quasar Astrometric Catalogue
No full textContext. In addition to being objects of intense interest for cosmology and studies of the evolution of the universe, quasars are used to define quasi-inertial positions on the celestial sphere. Since its first publication in 2009, the purpose of the Large Quasar Astrometric Catalogue (LQAC) has been to provide the largest catalogue of bona fide quasars suitable for astrometry, with the high-precision positions from Gaia becoming available in the fourth release (LQAC-4).
Aims. Following the same kind of procedure as in the previous releases of the LQAC, our aim is to compile the maximum number of recorded quasars, with optimised equatorial coordinates and fundamental information about their physical properties such as the redshift, the photometry, radio fluxes, morphological index.
Methods. New quasars from the SDSS DR16Q release were cross-matched with the precedent LQAC-5 compilation with a 1″ radius, which leads to 225 082 objects not present in the previous LQAC-5 release. Another cross-match was done with the Gaia Celestial Reference Frame 3 (GCRF3) catalogue, which is itself a compilation of a large number of quasars catalogues, accompanied with very accurate Gaia DR3 positions. Moreover, AGN’s coming from 19 individual extragalactic surveys not considered in the previous LQAC compilations are added.
Results. This new LQAC-6 release contains 2 073 099 objects, which is 3.5 times the number of objects recorded in the previous LQAC-5 release. Among them, 1 739 187 objects were found in common with the Gaia DR3, within a 1″ search radius. A large part of new objects belong to the GCRF3 catalogue, which gathers a total number of 1 614 218 objects.
Conclusions. The LQAC-6 delivers a nearly complete catalogue of quasars and AGN’s to the astronomical community, with the aim of giving their best equatorial coordinates with respect to the recent ICRF3 and with exhaustive additional information. For as much as 83.9% of the sample, these coordinates are extracted from the very recent Gaia DR3
Sidereal orientation of the Earth and stability of the VLBI celestial reference frame
No full textInternational audienceThe consideration of time stability of extragalactic radio sources observed by VLBI is shown to allow the realisation of more consistent celestial reference frames. The impact on the estimation of precession and nutation components is investigated over the time span 1984-2002. The precession correction to the IAU 2000 value that is obtained when excluding the unstable sources reaches 49 ± 5 mu as/year, to be compared to 12 ± 5 mu as/year using the current conventional celestial frame. The determination of the obliquity rate is unaffected and remains at the level of 27 ± 2 mu as/year. The observed correction to the 18.6-year nutation amplitude using the current conventional celestial frame ie sizeably corrupted by the unstable sources. After accounting for this effect, the estimations relative to both sets of reference radio sources confirm a discrepancy with the IAU 2000 nutation model with a total amplitude of 320 ± 100 mu as for the observed nutation in longitude, to be compared to the 80 mu as discrepancy found by Mathews et al. (2002, JGRB, 107, 1029). The discrepancy in obliquity amounts to 50 ± 16 mu as. The effect of source instability is shown to have an impact on the determination of universal time at the one microsecond level. The high and medium frequency nutation terms (up to periods of a few years) are impacted only in the early years of the program. Chapter 7 concerning the observation of the core and inner core free nutations is paralleled by a twin paper (Dehant et al. 2005, A&A, 438, 1149) that proposes a theoretical development for their atmospheric and oceanic excitation
Analysis strategy issues for the maintenance of the ICRF axes
No full textInternational audienceIn preparation for the evolution of the definition of the VLBI-based International Celestial Reference Frame (ICRF), various elements of the analysis strategy are investigated, such as reference source selection and the impact of the status of the terrestrial reference frame in the data analysis model. We conclude that including the determination of both the celestial and the terrestrial reference frames in the analysis does not affect the quality of the celestial reference frame. The determination of precession and nutation components is not affected by the status of the terrestrial reference frame but it is affected by the selection of reference radio sources. Extending an earlier study, three lists of reference sources based on progressively larger tolerances are proposed. They include 181, 225 and 247 objects, respectively
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