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Gaia Data Release 1: Reference frame and optical properties of ICRF sources

Mignard, F. and Klioner, S. and Lindegren, L. and Bastian, U. and Bombrun, A. and Hernández, J. and Hobbs, D. and Lammers, U. and Michalik, D. and Ramos-Lerate, M. and Biermann, M. and Butkevich, A. and Comoretto, G. and Joliet, E. and Holl, B. and Hutton, A. and Parsons, P. and Steidelmüller, H. and Andrei, A. and Bourda, G. and Charlot, P. (2016) Gaia Data Release 1: Reference frame and optical properties of ICRF sources. Astronomy and Astrophysics, 595 . Art. No. A5. ISSN 0004-6361.

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Context. As part of the data processing for Gaia Data Release 1 (Gaia DR1) a special astrometric solution was computed, the so-called auxiliary quasar solution. This gives positions for selected extragalactic objects, including radio sources in the second realisation of the International Celestial Reference Frame (ICRF2) that have optical counterparts bright enough to be observed with Gaia. A subset of these positions was used to align the positional reference frame of Gaia DR1 with the ICRF2. Although the auxiliary quasar solution was important for internal validation and calibration purposes, the resulting positions are in general not published in Gaia DR1. Aims. We describe the properties of the Gaia auxiliary quasar solution for a subset of sources matched to ICRF2, and compare their optical and radio positions at the sub-mas level. Methods. Descriptive statistics are used to characterise the optical data for the ICRF sources and the optical-radio differences. The most discrepant cases are examined using online resources to find possible alternative explanations than a physical optical-radio offset of the quasars. Results. In the auxiliary quasar solution 2191 sources have good optical positions matched to ICRF2 sources with high probability. Their formal standard errors are better than 0.76 milliarcsec (mas) for 50% of the sources and better than 3.35 mas for 90%. Optical magnitudes are obtained in Gaia’s unfiltered photometric G band. The Gaia results for these sources are given as a separate table in Gaia DR1. The comparison with the radio positions of the defining sources shows no systematic differences larger than a few tenths of a mas. The fraction of questionable solutions, not readily accounted for by the statistics, is less than 6%. Normalised differences have extended tails requiring case-by-case investigations for around 100 sources, but we have not seen any difference indisputably linked to an optical-radio offset in the sources. Conclusions. With less than a quarter of the data expected from the nominal mission it has been possible to obtain positions at the sub-mas level for most of the ICRF sources having an optical counterpart brighter than 20.5 mag.

Item Type:Article
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Additional Information:© ESO, 2016. Received 16 August 2016 / Accepted 20 September 2016. This work has made use of data from the ESA space mission Gaia, processed by the Gaia Data Processing and Analysis Consortium (DPAC). This research has made use of Aladin sky atlas and the Simbad database developed at CDS, Strasbourg Observatory, France. We are grateful to the developers of TOPCAT (Taylor 2005) for their software. Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The Gaia mission website is The authors are members of the Gaia DPAC. This work has been supported by the European Space Agency in the framework of the Gaia project; the Centre National d’Études Spatiales (CNES); the German Aerospace Agency DLR under grants 50QG0501, 50QG1401 50QG0601, 50QG0901, and 50QG1402; and the Swedish National Space Board. The authors are very grateful to Chris Jacobs for his constructive review and thoughtful remarks, which helped to improve the paper.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Gaia Multilateral AgreementUNSPECIFIED
European Space Agency (ESA)UNSPECIFIED
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Deutsches Zentrum für Luft- und Raumfahrt (DLR)50QG0501
Deutsches Zentrum für Luft- und Raumfahrt (DLR)50QG1401
Deutsches Zentrum für Luft- und Raumfahrt (DLR)50QG0601
Deutsches Zentrum für Luft- und Raumfahrt (DLR)50QG0901
Deutsches Zentrum für Luft- und Raumfahrt (DLR)50QG1402
Swedish National Space Board (SNSB)UNSPECIFIED
Subject Keywords:astrometry / reference systems / quasars: general
Record Number:CaltechAUTHORS:20161222-103443462
Persistent URL:
Official Citation:Gaia Data Release 1 - Reference frame and optical properties of ICRF sources F. Mignard, S. Klioner, L. Lindegren, U. Bastian, A. Bombrun, J. Hernández, D. Hobbs, U. Lammers, D. Michalik, M. Ramos-Lerate, M. Biermann, A. Butkevich, G. Comoretto, E. Joliet, B. Holl, A. Hutton, P. Parsons, H. Steidelmüller, A. Andrei, G. Bourda and P. Charlot A&A, 595 (2016) A5 DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:73143
Deposited By: George Porter
Deposited On:22 Dec 2016 19:18
Last Modified:03 Oct 2019 16:24

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