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Zusammenfassung:
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One of the main objectives of the Gaia mission of the European Space Agency is
to construct a celestial reference frame at the wavelengths of the optical domain, Gaia CRF.
This frame needs a link to the International Celestial Reference Frame – ICRF which is fixed
with respect to distant objects (quasars). The objects serving for the purpose of linking are
required to be visible in both domains (optical and radio). A set of 47 such objects has been
proposed and included which in the radio domain have no detected extended emission.
The mentioned objects are active galactic nuclei (AGN) the brightness of which varies
over the whole electromagnetic spectrum. The brightness change may be due to activity in
different AGN regions, but also to external factors. Such variations can lead to changes in
the photocentre position and, consequently, to changes of the object coordinates. In order
to establish which objects are suitable for linking these two frames we have examined the
brightness variation in the optical domain. The objects have been observed from 2013 in the
V and R bands. We have analysed the brightness, colour (V − R) and optical spectral index
(α).
It has been established that for the majority of objects the brightness is variable, or possibly
variable. Almost 15% of all objects have significant changes in their brightness (more than 1
mag), only ∼10% are stable with minor brightness changes of ∼0.3 mag. The results concerning
the change analysis of the colour and α are also presented. Based on these results 17 objects
are chosen as suitable for linking ICRF to Gaia CRF. The results of the analysis, as well as the
observed values, are essential for the examination of these objects because of their importance in
astrometry, also in astrophysics. These data are relevant to a better understanding of formation
and evolution of galaxies. |