Optical monitoring of extragalactic sources for linking the ICRF and the future Gaia celestial reference frame

I. Variability of ICRF sources^⋆

¹ Observatoire de Paris – SYRTE, CNRS/UMR 8630 & Université Pierre et Marie Curie, 75005 Paris, France
e-mail: Francois.Taris@obspm.fr
² Observatório Nacional/MCT, 20921-400 Rio de Janeiro, Brazil
³ Obervatorio do Valongo, UFRJ, Brazil
⁴ Osservatorio Astronomico di Torino, INAF, 10025 Pino Torinese, Italy
⁵ Shangai Astronomical Observatory, CAS, 200030 Shanghai, PR China
⁶ Université de Toulouse, UPS/OMP, IRAP, 31400 Toulouse, France
⁷ CNRS, IRAP, 14 avenue Édouard Belin, 31400 Toulouse, France
⁸ Observatoire de Paris – IMCCE, 75005 Paris, France
⁹ CICGE, Faculdade de Ciências da Universidade do Porto, 4150-564 Porto, Portugal
¹⁰ SIM, Faculdade de Ciências da Universidade de Lisboa, 1749-016 Lisbon, Portugal
¹¹ Université de Bordeaux, OASU, 2 rue de l’Observatoire, BP 89, 33271 Floirac Cedex, France
¹² CNRS, UMR 5804, LAB, 2 rue de l’Observatoire, BP 89, 33271 Floirac Cedex, France
¹³ School of Physics, University of Western Australia, Crawley WA 6009, Australia

Received: 25 May 2012
Accepted: 3 December 2012

Abstract

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.