Gaia Celestial Reference Frame Research Articles

The Astrometric Performance of the China Space Station Telescope (CSST) Sky Survey in Extending the Gaia Celestial Reference Frame

The multi-color imaging sky survey conducted by the China Space Station Telescope (CSST) holds significant promise for advancing the development of the celestial reference frame. In this study, we focus on assessing the astrometric performance of the CSST celestial reference frame (CSST-CRF) in extending the Gaia Celestial Reference Frame 3 (Gaia-CRF3). First, the orientation precision of the CSST reference frame is evaluated using a simulated set of extragalactic sources with CSST g magnitudes ranging from 18 to 25 mag. The estimated orientation uncertainty caused by random error insignificantly affects the alignment between Gaia-CRF3 and the CSST-CRF. Then, the systematic effect of incomplete CSST sky coverage on the alignment between CSST-CRF and Gaia-CRF3 is discussed by analyzing the differences between the subset of Gaia-CRF3 in the CSST observation region (Gaia-CRF3′) and Gaia-CRF3 as a whole. Using the third International Celestial Reference Frame (ICRF3) S/X band as an intermediate reference frame, the orientation offset between Gaia-CRF3′ and Gaia-CRF3 is estimated to be 20 μas. This offset is marginally larger than the orientation offset between Gaia-CRF3 and the ICRF3, approximately 15 μas. The residual spin and glide rate of Gaia-CRF3′ are derived from the proper motions, consistent with that of Gaia-CRF3 within the formal error. Finally, we explore the role of CSST in establishing a multi-band celestial reference frame by comparing its limiting magnitude and observation accuracy with existing catalogs in the infrared and ultraviolet bands. Thanks to its broad wavelength coverage and high-precision measurements, CSST is well-positioned to make significant contributions to the development of a multi-band celestial reference frame.

Time-lapse Very Long Baseline Interferometry Imaging of the Close Active Binary HR 1099

We report multiepoch astrometric very long baseline interferometry observations of the chromospherically active binary HR 1099 (V711 Tau, HD 22468) at six epochs over 63 days using the Very Long Baseline Array at 22.2 GHz. We determined hourly radio positions at each epoch with a positional uncertainty significantly smaller than the component separation. The aggregate radio positions at all epochs define an ellipse in the comoving reference frame with an inclination i=39.°5−3.°5+3.°6 and longitude of ascending node Ω = 212° ± 22°. The ellipse center is offset from the Third Gaia Celestial Reference Frame position by Δα = −0.81−0.37+0.25 , Δδ = 0.45−0.25+0.23 mas. All radio centroids are well displaced from the binary center of mass at all epochs, ruling out emission from the interbinary region. We examined the motion of the radio centroids within each epoch by comparing hourly positions over several hours. The measured speeds were not statistically significant for five of the six epochs, with 2σ upper limits in the range 200–1000 km s−1. However, for one flaring epoch, there was a 3σ detection v ⊥ = 228 ± 85 km s−1. This speed is comparable to the mean speed of observed coronal mass ejections on the Sun.

VLBI astrometry of radio stars to link radio and optical celestial reference frames: observing strategies

ABSTRACT The Gaia celestial reference frame (Gaia-CRF) will benefit from a close assessment with independent methods, such as Very Long Baseline Interferometry (VLBI) measurements of radio stars at bright magnitudes. However, obtaining full astrometric parameters for each radio star through VLBI measurements demands a significant amount of observation time. This study proposes an efficient observing strategy that acquires double-epoch VLBI positions to measure the positions and proper motions of radio stars at a reduced cost. The solution for CRF link compatible with individual VLBI position measurements is introduced, and the optimized observing epoch scheduling is discussed. Applying this solution to observational data yields results sensitive to sample increase or decrease, yet they remain consistently in line with the literature at the 1σ level. This suggests the potential for improvement with a larger sample size. Simulations show that the double-epoch strategy reduces CRF link parameter uncertainties by over 30 per cent compared to the five-parameter strategy.

LQAC-6: Sixth Release of the Large Quasar Astrometric Catalogue

Context. 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 Conclusion 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 $83.9 <!PCT!>$ of the sample, these coordinates are extracted from the very recent Gaia DR3.

Towards a robust estimation of orientation parameters between ICRF and Gaia celestial reference frames

ABSTRACT An analysis of the source position differences between very long baseline interferometry-based ICRF and Gaia-celestial reference frame catalogues is a key step in assessing their systematic errors and determining their mutual orientation. One of the main factors that limits the accuracy of determination of the orientation parameters between two frames is the impact of outliers. To mitigate this effect, a new method is proposed based on pixelization data over the equal-area cells, followed by median filtering of the data in each cell. After this, a new data set is formed, consisting of data points near-uniformly distributed over the sphere. The vector spherical harmonics decomposition is then applied to this data to finally compute the orientation parameters between ICRF and Gaia frames. To validate the proposed approach, a comparison was made of the ICRF3-SX and Gaia DR2 catalogues using several methods for outliers removal. The results of this work showed that the proposed method is practically insensitive to outliers and thus provides much more robust results of catalogues comparison than the methods used so far. This conclusion was confirmed by analogous test comparison of the Gaia DR2 and OCARS catalogues.

The Northern Extragalactic WISE × Pan-STARRS (NEWS) catalogue

This study involves two photometric catalogues, AllWISE and Pan-STARRS Data Release 1, which were cross-matched to identify extragalactic objects among the common sources of these catalogues. To separate galaxies and quasars from stars, we created a machine-learning model that is trained on photometric (in fact, colour-based) information from the optical and infrared wavelength ranges. The model is based on three important procedures: the construction of the autoencoder artificial neural network, separation of galaxies and quasars from stars with a support vector machine (SVM) classifier, and cleaning of the AllWISE × PS1 sample to remove sources with abnormal colour indices using a one-class SVM. As a training sample, we employed a set of spectroscopically confirmed sources from the Sloan Digital Sky Survey Data Release 14. Having applied the classification model to the data of crossing the AllWISE and Pan-STARRS DR1 samples, we created the Northern Extragalactic WISE × Pan-STARRS (NEWS) catalogue, containing 40 million extragalactic objects and covering 3/4 of celestial sphere up tog = 23m. Several independent classification quality tests, namely, the astrometric test along with others based on the use of data from spectroscopic surveys show similar results and indicate a high purity (∼98.0%) and completeness (> 98%) for the NEWS catalogue within the magnitude range of 19.0m < g < 22.5m. The classification quality still retains quite acceptable levels of 70% for purity and 97% for completeness for the brightest and faintest objects from this magnitude range. In addition, validation with external data sets has demonstrated the need for using only those sources in the NEWS catalogue that are outside the zone with the enhanced extinction. We show that the number of quasars from the NEWS catalogue identified inGaiaDR2 exceeds the number of quasars previously identified inGaiaDR2 with the use of the AllWISEAGN catalogue. These quasars may be used in future as an additional sample for testing and anchoring theGaiaCelestial Reference Frame.

Optical variability of some quasars important to ICRF-Gaia CRF link

The Gaia optical observations started a few years ago. As a result, the Gaia Celestial Reference Frame (Gaia CRF) should replace the International CRF (ICRF). This could be done via extragalactic radio sources (mostly quasars - QSOs) visible in optical domain. During about 2.5 years (for the period July 2016 { April 2019) of our original observations of some QSOs outside ICRF list we collected observations in the V and R bands for five objects and their 30 comparison stars. Photometry stability of these objects is of importance for astrometry and the mentioned link. Because of it we did investigation of brightness variability of objects and their suitable comparison stars, and the F{test was performed. As a result, only the brightness of one object (1556+335) does not show variability. Other four objects were examined to determine the quasiperiods of their light curves using the method of Least Squares: 1535+231 (3.1 years in V, and 1.7 and 5.2 years in R filter), 1607+604 (2.7 years in V, and 1.3 and 2.3 years in R), 1722+119 (1.3 and 2.7 years in V, and 1.3 and 5.3 years in R), and 1741+597 (6.5 years in V, and 1.3 and 4.0 years in R). After a similar analysis of variability of comparison stars, the conclusion is that all of them are useful for differential photometry. Also, we provide our finding charts for these objects with suitable comparison stars.

Connecting VLBI and Gaia Celestial Reference Frames

The current state of the link problem between radio and optical celestial reference frames is considered. The main objectives of the investigations in this direction during the next few years are the preparation of a comparison and the mutual orientation and rotation between the optical {\it Gaia} Celestial Reference Frame (GCRF) and the 3rd generation radio International Celestial Reference Frame (ICRF3), obtained from VLBI observations. Both systems, ideally, should be a realization of the ICRS (International Celestial Reference System) at micro-arcsecond level accuracy. Therefore, the link accuracy between the ICRF and GCRF should be obtained with similar error level, which is not a trivial task due to relatively large systematic and random errors in source positions at different frequency bands. In this paper, a brief overview of recent work on the GCRF--ICRF link is presented. Additional possibilities to improve the GCRF--ICRF link accuracy are discussed. The suggestion is made to use astrometric radio sources with optical magnitude to 20$^m$ rather than to 18$^m$ as currently planned for the GCRF--ICRF link. In addition, the use of radio stars is also a prospective method to obtain independent and accurate orientation between the Gaia frame and the ICRF.

Using radio stars to link theGaiaand VLBI reference frames

A possible method for linking the optical Gaia Celestial Reference Frame (GCRF) to the VLBI-based International Celestial Reference Frame (ICRF) is to use radio stars in a manner similar to that in the linking of the Hipparcos Celestial Reference Frame (HCRF) to ICRF. In this work, an obtainable accuracy of the orientation angles between GCRF and ICRF frames was estimated by Monte Carlo simulation. If the uncertainties in the radio star positions obtained by VLBI are in the range of 0.1-4 mas and those obtained by Gaia are in the range of 0.005-0.4 mas, the orientation angle uncertainties are 0.018-0.72 mas if 46 radio stars are used, 0.013-0.51 mas if 92 radio stars are used, and 0.010-0.41~mas if 138 radio stars are used. The general conclusion from this study is that a properly organized VLBI programme for radio star observation with a reasonable load on the VLBI network can allow for the realization of GCRF-ICRF link with an error of about 0.1 mas.

Long-termRandV-band monitoring of some suitable targets for the link between ICRF and the futureGaiacelestial reference frame

Context. The Gaia astrometric mission of the European Space Agency was launched on December 2013. It will provide a catalog of 500 000 quasars. Some of these targets will be chosen to build an optical reference system that will be linked to the International Celestial Reference Frame (ICRF). The astrometric coordinates of these sources will have roughly the same uncertainty at both optical and radio wavelengths, and it is then mandatory to observe a common set of targets to build the link. In the ICRF, some targets have been chosen because of their pointlikeness. They are quoted as defining sources, and they ensure very good uncertainty about their astrometric coordinates. At optical wavelengths, a comparable uncertainty could be achieved for targets that do not exhibit strong astrophysical phenomena, which is a potential source of photocenter flickering. A signature of these phenomena is a magnitude variation at optical wavelengths.Aims. The goal of this work is to present the time series of 14 targets suitable for the link between the ICRF and the future Gaia Celestial Reference Frame. The observations have been done systematically by robotic telescopes in France and Chile once every two nights since 2011 and in two filters. These time series are analyzed to search for periodic or quasi-periodic phenomena that must be taken into account when computing the uncertainty about the astrometric coordinates.Methods. Two independent methods were used in this work to analyze the time series. We used the CLEAN algorithm to compare the frequency obtained to those given by the Lomb-Scargle method. It avoids misinterpreting the frequency peaks given in the periodograms.Results. For the 14 targets we determine some periods with a confidence level above 90% in each case. Some of the periods found in this work were not previously known. For the others, we did a comparative study of the periods previously studied by others and always confirm their values. All the periods given by the two methods are in good agreement with the difference always below 7.9% in the worst case. Finally we also present the structure functions for our two sets of objects (BL Lac and Seyfert galaxies).Conclusions. For all but one target, we find variations that could be the signature of periodic or quasi-periodic phenomena. Our time series could allow to bring some constraints on astrophysical models that could explain such variations. Binary black hole system, instabilities in the jet or in the accretion disk, changes in the torus structure are some of these astrophysical phenomena. They must be kept in mind when evaluating the uncertainty about the astrometric position of the targets suitable for the link between reference systems in a conservative way.

Optical monitoring of extragalactic sources for linking the ICRF and the futureGaiacelestial reference frame

Context. The astrometric mission Gaia of the European Space Agency is scheduled to be launched in 2013. It will provide an astrometric catalog of 500 000 extragalactic sources that could be the basis of a new optical reference frame after the Hipparcos satellite one. On the other hand, the current International Celestial Reference Frame (ICRF) is based on observations of extragalactic sources at radio wavelength. The astrometric coordinates of sources in these two reference systems will have roughly the same uncertainty. It is then mandatory to observe a set of common targets at both optical and radio wavelengths to link the ICRF with what could be called the Gaia Celestial Reference Frame (GCRF). Aims. The goal of this work is to observe a first set of 70 extragalactic sources at optical wavelengths that could achieve the link with the ICRF. Variations in the light curves of these targets are connected with astrophysical processes that could produce displacements of the optical photocenter. Such displacements, if they exist, are critical in the framework of the link of reference systems. Methods. Four telescopes were used to observe the targets at optical wavelengths. Two of them are located in France, one in Chile, and the last one in Australia. First observations were carried out during one year and a half in the R and V bands. A new method of characterizing the compactness of the targets was applied to the images obtained. Results. This paper presents results for the optical monitoring of extragalactic sources suitable for linking reference systems. We show that a large number of targets in our set are variable at the two observational wavelengths. A short presentation of each object is given, along with some references to earlier photometric studies. A morphological index is defined and applied to the 5000 images obtained during the observation campaign. Conclusions. This work fits into a more general project of astrophotometric and astrophysical studies of extragalactic radiosources in the framework of the reference systems. It brings to the astrometric community some information at optical wavelengths about a set of targets that could be used for the link between the radio ICRF and the future GCRF.

Astrometric suitability of optically-bright ICRF sources for the alignment with the future Gaia celestial reference frame

The ICRF, currently based on the position of 717 extragalactic radio sources observed by VLBI, is the fundamental celestial reference frame adopted by the IAU in 1997. Within the next 10 years, the European space astrometry mission Gaia, to be launched by 2011, will permit determination of the extragalactic reference frame directly in the visible for the first time. Aligning these two frames with the highest accuracy will therefore be very important in the future for ensuring consistency between the measured radio and optical positions. This paper is aimed at evaluating the current astrometric suitability of the individual ICRF radio sources which are considered appropriate for the alignment with the future Gaia frame. To this purpose, we cross-identified the ICRF and the optical catalog Veron-Cetty and Veron (2006), to identify the optically-bright ICRF sources that will be positioned with the highest accuracy with Gaia. Then we investigated the astrometric suitability of these sources by examining their VLBI brightness distribution. We identified 243 candidate ICRF sources for the alignment with the Gaia frame (with an optical counterpart brighter than the apparent magnitude 18), but only 70 of these (10% of the ICRF sources) are found to have the necessary high astrometric quality (i.e. a brightness distribution that is compact enough) for this link. Additionally, it was found that the QSOs that will have the most accurate positions in the Gaia frame tend to have less-accurate VLBI positions, most probably because of their physical structures. Altogether, this indicates that identifying other high-quality VLBI radio sources suitable for the alignment with the future Gaia frame is mandatory.

A Gaia oriented analysis of a large sample of quasars

AbstractGaia photometric capabilities should distinguish quasars to a high degree of certainty. With this, they should also be able to deliver a clean sample of quasars with a negligible trace of stellar contaminants. However, a purely photometric sample could miss a non negligible percentage of ICRF sources counterparts - and this interface is required to align with the ICRS and de-rotate the GCRF (Gaia Celestial Reference Frame), on grounds of continuity. To prepare a minimum clean sample forming the initial quasar catalogue for the Gaia mission, an all sky ensemble was formed containing 128,257 candidates. Among them there is at least one redshift determination for 98.75%, and at least one magnitude determination for 99.20% of the targets. The sources were collected from different optical and radio lists. We analyze the redshift, magnitude, and color distributions, their relationships, as well as their degree of completeness.Complementary, the candidate sources enable to form an optical representation of the ICRS from first principles, namely, kinematically non-rotating with respect to the ensemble of distant extragalactic objects, aligned to the mean equator and dynamical equinox of J2000, and realized by a list of adopted coordinates of extragalactic sources.