Characterizing the astrometric instability of extragalactic radio source positions measured with geodetic VLBI

International audienceContext. Geodetic very long baseline interferometry (VLBI) has been used to observe extragalactic radio sources for more than 40 yr. The absolute source positions derived from the VLBI measurements serve as a basis to define the International Celestial Reference Frame (ICRF). Despite being located at cosmological distances, an increasing number of these sources are found to show position instabilities, as revealed by the accumulation of VLBI data over the years.Aims. We investigate how to characterize the astrometric source position variations, as measured with geodetic VLBI data, in order to determine whether these variations occur along random or preferential directions. The sample of sources used for this purpose is made up of the 215 most observed ICRF sources.Methods. Based on the geodetic VLBI data set, we derived source coordinate time series to map the apparent trajectory drawn by the successively measured positions of each source in the plane of the sky. We then converted the coordinate time series into a set of vectors and used the direction of these vectors to calculate a probability density function (PDF) for the direction of variation of the source position. For each source, a model that matches the PDF and that comprises the smallest number of Gaussian components possible was further adjusted. The resulting components then identify the preferred directions of variation for the source position.Results. We found that more than one-half of the sources (56%) in our sample may be characterized by at least one preferred direction. Among these, about three-quarters are characterized by a unique direction, while the remaining sources show multiple preferred directions. The analysis of the distribution of these directions reveals an excess along the declination axis that is attributed to a VLBI network effect. Whether single or multiple, the identified preferred directions are likely due to source-intrinsic physical phenomena

Realization of Celestial Reference Frames using the Allan Variance Classification

Recently te{GattanoLambertLeBail2018}, a new classification of VLBI radio sources was built on the basis of their astrometric stability revealed by the use of the Allan standard deviation. In such a classification, sources are divided into three groups depending on the nature of the noise content in the astrometric time series. The global level of noise then orders sources within each group. In this proceedings, we present several strategies on the basis of this classification to realize celestial reference frames, i.e. for selecting the set of defining sources used to define the fundamental axes of the frame. This set of sources is usually constrained in the data reduction by a no-net rotation constraint. Using two tools developed to determine the stability of realized frames, one that analyzes the stability of the annual realizations of a given frame and another that analyzes the coherence of random sub-frames, we determine the best usage of this classification

Insight into Astrophysical Phenomena from VLBI Source Position Instabilities

Most of the radio sources observed by VLBI, some used as defining sources in the International Celestial Reference Frame [ICRF2], show instabilities in position. These instabilities may be caused by astrophysical phenomena occurring in the central VLBI region of these objects (i.e., active galactic nuclei). On this basis, we have begun to characterize the signal included in the available VLBI position time series. Often, position instabilities happen along a preferred direction. There are cases, however, where two directions are distinguishable. The first scenario is consistent with a regular emergence of jet components from the VLBI core; hence, causing shifts of the radio emission centroid. On the other hand, the second scenario may give clues to the presence of a second black hole within the system that has its own activity offset from that of the first black hole. Comparing these directions with the orientation derived from radio-optical position offset brings further insights into astrophysical phenomena within active galactic nuclei

A whole sky study of quasars known population starting from the LQAC-2 compiled catalogue

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Extragalactic radio source stability and VLBI celestial reference frame: insights from the Allan standard deviation

International audienceAims. We investigate the composition of the noise in coordinate time series of several hundreds of extragalactic radio sources monitored by the geodetic VLBI program since 1979. The noise type is identified at all available timescales longer than one year, following the observational history of the source.Methods. We computed the Allan standard deviation of coordinate time series and developed a Monte Carlo test to evaluate the influence of the irregular sampling and error on data onto the noise type identification. We classified the radio sources into three categories depending on their type of noise and taking into account the dominating noise at different timescales: from the category AV0, which contains sources with a stable behavior at all timescales, to the category AV2, which contains sources whose coordinates are dominated by random walks at the longest timescales.Results. We found that almost no source exhibited “idealized” white noise. Only 5% of the 647 sources we studied belong to the category AV0 (stable sources). Moreover, we found that this class contains sources with relatively short observational histories, suggesting that after some years, a source whose astrometric position has shown a stable behavior is likely to become unstable. This questions the existence of the stable source paradigm and adds complementary information in the crucial task of selecting sources on which to base the axes of the celestial reference frame

Earth's core and inner-core resonances from analysis of VLBI nutation and superconducting gravimeter data

International audienceGeophysical parameters of the deep Earth’s interior can be evaluated through the resonanceeffects associated with the core and inner-core wobbles on the forced nutations of the Earth’sfigure axis, as observed by very long baseline interferometry (VLBI), or on the diurnal tidalwaves, retrieved from the time-varying surface gravity recorded by superconducting gravime-ters (SGs). In this paper, we inverse for the rotational mode parameters from both techniquesto retrieve geophysical parameters of the deep Earth. We analyse surface gravity data from 15SG stations and VLBI delays accumulated over the last 35 yr. We show existing correlationsbetween several basic Earth parameters and then decide to inverse for the rotational modesparameters. We employ a Bayesian inversion based on the Metropolis-Hastings algorithm witha Markov-chain Monte Carlo method. We obtain estimates of the free core nutation resonantperiod and quality factor that are consistent for both techniques. We also attempt an inversionfor the free inner-core nutation (FICN) resonant period from gravity data. The most probablesolution gives a period close to the annual prograde term (or S1 tide). However the 95 per centconfidence interval extends the possible values between roughly 28 and 725 d for gravity, andfrom 362 to 414 d from nutation data, depending on the prior bounds. The precisions of theestimated long-period nutation and respective small diurnal tidal constituents are hence notaccurate enough for a correct determination of the FICN complex frequency

LQAC-4: Fourth release of the Large Quasar Astrometric Catalogue

Context. From an astrometric point of view, quasars constitute the best and almost ideal reference objects in the celestial sphere, with a priori no significant proper motion. Since the third release of the Large Quasar Astrometric Catalogue (LQAC-3), a large number of quasars have been discovered, in particular those coming from the DR12Q release of the SDSS. Moreover, for cross-matched objects, we have taken advantage of the very accurate determinations of the quasars identified within the recent Gaia DR1 catalogue. Aims. Following the same procedure as in the three previous releases of the LQAC, our aim is to compile the large majority of all the quasars recorded so far. Our goal is to record their best coordinates and substantial information concerning their physical properties such as the redshift as well as multi-bands apparent and absolute magnitudes. Emphasis is given to the results of the cross-matches with the Gaia DR1 catalogue. Methods. New quasars coming from the DR12Q release were cross-matched with the precedent LQAC-3 compilation with a 1′′ search radius, in order to add the objects without counterpart to the LQAC-4 compilation. A similar cross-match was done with Gaia DR1 to identify the known quasars detected by Gaia. This enables one to improve significantly the positioning of these objects, and in parallel to study the astrometric performance of the individual catalogues of the LQAC-4 compilation. Finally, a new method was used to determine absolute magnitudes. Results. Our final catalogue, called LQAC-4, contains 443 725 objects. This is roughly 37.82% more than the number of objects recorded in the LQAC-3. Among them, 249 071 were found in common with the Gaia DR1, with a 1′′ search radius. That corresponds to 56.13% of the whole population in the compilation. Conclusions. The LQAC-4 delivers to the astronomical community a nearly complete catalogue of spectroscopically confirmed quasars (including a small proportion of compact AGNs), with the aim of giving their best equatorial coordinates with respect to the ICRF2 and with exhaustive additional information. For more than 50% of the sample, these coordinates come from the very recent Gaia DR1

Measurement of Earth's Nutation by VLBI: Direct Estimates from VLBI Delays and a Discussion on the Error

Accurate measurements of the Earth nutation by VLBI provide insights into the deformability of and the coupling mechanisms at the core-mantle and core-inner core boundaries. We propose here to adjust nutation amplitudes directly to VLBI delays, as opposed to the traditional method consisting of fitting nutation amplitudes to time-domain nutation series. However, the complexity of the VLBI analysis chain makes the formal error on the parameters somehow obscure and disconnected from a realistic error based on, e.g., empirical tests of robustness and errors on models. In this work, we address some striking differences between formal and empirical errors

A compilation of 592 809 objects with 398 697 Gaia counterparts

Context. In addition to their great astrophysical interest, quasars represent quasi-ideal reference objects in the celestial sphere with, a priori, a lack of significant proper motion. Since the fourth release of the Large Quasar Astrometric Catalogue (LQAC-4), a large number of quasars have been discovered, in particular those coming from the DR14Q release of the SDSS. With the advent of the Gaia Data Release 2 (DR2), it is now also possible to fold in extremely accurate quasar positions.Aims. Following the same procedure as in the previous releases of the LQAC, our aim is to compile the large majority of the recorded quasars, with their best estimated coordinates and substantial information about their physical properties such as the redshift, multi-bands apparent, and absolute magnitudes. Emphasis is given to the results of the cross-matches with the Gaia DR2 catalogue, which considerably increases the positional accuracy.Methods. New quasars from the SDSS DR14Q release were cross-matched with the precedent LQAC-4 compilation with a 1″ search radius, which leads to 149 084 objects not present in the previous LQAC-4 release. Another cross-match was done with the Gaia DR2 catalogue, which enables us to considerably improve the positioning of these objects. For the first time, parallaxes and proper motions from the DR2, when available, are added to our compilation. Furthermore, a cross-identification of the LQAC-5 with the AllWISE survey gives additional mid-infrared information for an important percentage of objects.Results. Our final catalogue, namely the LQAC-5, contains 592 809 quasars. This represents roughly a 34% increase with respect to the number of objects recorded in the LQAC-4. Among them, 398 697 objects were found in common with the Gaia DR2, within a 1″ search radius. That corresponds to 67.26% of the whole population of the compilation.Conclusion. The LQAC-5 delivers a nearly complete catalogue of spectroscopically confirmed quasars (including a small proportion of 14 126 compact AGN’s) to the astronomical community, with the aim of giving their best equatorial coordinates with respect to the ICRF2 and with exhaustive additional information. For more than 50% of the sample, these coordinates are extracted from the very recent Gaia DR2

The third release of the Large Quasar Astrometric Catalog (LQAC-3): a compilation of 321 957 objects

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