Volume 526, February 2011
|Number of page(s)||16|
|Section||Celestial mechanics and astrometry|
|Published online||15 December 2010|
Observatoire de Paris, Systèmes de Référence Temps Espace (SYRTE),
2 Observatório Nacional/MCT, Rio de Janeiro, Brasil
3 Observatório do Valongo/UFRJ, Rio de Janeiro, Brasil
4 Osservatorio Astronomico di Torino/INAF, Torino, Italy
5 CICGE-Fac. Sciences Univ. Porto, Porto, Portugal
Received: 29 July 2010
Accepted: 12 October 2010
Context. The current conventional realization of the ICRS (International Celestial Reference System) is, in the radio wavelength, the International Celestial Reference Frame 2 (ICRF2). The individual positions of the defining sources have been found to have accuracies better than 1 milliarcsecond (mas). In 2012, the European astrometric satellite Gaia will be launched. This mission will provide an astrometric catalog of an estimated number of 500 000 QSOs. The uncertainty in the coordinates is anticipated to be 200 microarcsecond (μas) for the magnitude = 20. If this were achieved, the ICRF and the Gaia related reference frame could be related with a μas accuracy.
Aims. The goal of this work is both to measure the photometric variability of a set of quasars in a given field, and search wether this variability can be related to an astrometric instability characterized by a motion of the quasar photocenter. If this correlation existed for some given QSO, then it would be inadequate to materialize the Gaia extragalactic reference frame at the level of confidence required, i.e. the sub-milliarcsecond one. This should be an important result in the scope of the Gaia mission.
Methods. We use QSO CCD images obtained over 4.5 years with the Canada France Hawaï Telescope (CFHT) in the framework of the CFHT-Legacy Survey (CFHT-LS). The pictures were analysed with both the SExtractor software and customised codes to perform a photometric calibration together with an astrometric one. A total of 41 QSOs in the Deep 2 field were analysed. Magnitude variations during more than 50 months are given at three different bandwiths G, R, and I. Among the set above, 5 quasars were chosen to test the ties between the postion of their centroid and their magnitude variations. For one of these 5 QSOs, the proximity of a neighbouring star allows the comparison between the PSFs.
Results. We clearly show significant photometric variations reaching sometimes more than one magnitude, for a good proportion of the 41 quasars in our sample. We show that these variations often occur within a few months, and that the correlation between the photometric curves in the three bands, G, R and I is obvious. As a second important result, we show that with a reasonably high probability, photometric variations for one quasar in our sample are accompagnied by substantial modification of its PSF.
Key words: reference systems / astrometry / quasars: general
This paper is dedicated to the memory of Anne-Marie Gontier (1966-2010). A.-M. Gontier was an expert in the field of Earth rotation, reference systems and the modeling, analysis, and processing of VLBI observations for astrometric and geodetic applications.
© ESO, 2010
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.