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In extending our spectroscopic program, which targets sources drawn from the International Celestial Reference Frame (ICRF) Catalog, we have obtained spectra for about 160 compact flat-spectrum radio sources and determined redshifts for 112…
Continuing our program of spectroscopic observations of ICRF sources, we present redshifts for 120 quasars and radio galaxies. Data were obtained with five telescopes: the 3.58m ESO New Technology Telescope (NTT), the two 8.2m Gemini…
A new list of physical characteristics of 4261 astrometric radio sources, including all 717 ICRF-Ext.2 sources has been compiled. Comparison of our data of optical characteristics with the official International Earth Rotation and Reference…
The current realization of the International Celestial Reference Frame (ICRF) comprises a total of 717 extragalactic radio sources distributed over the entire sky. An observing program has been developed to densify the ICRF in the northern…
The third iteration of the International Celestial Reference Frame (ICRF3) is made up of 4536 quasars observed at S/X bands using Very Long baseline Interferometry (VLBI). These sources are high redshift quasars, typically between $1<z<2$,…
The space astrometry mission GAIA will construct a dense optical QSO-based celestial reference frame. For consistency between the optical and radio positions, it will be important to align the GAIA frame and the International Celestial…
From observations with the 6-m BTA telescope at SAO RAS, we have determined spectroscopic redshifts of seven optical objects whose coordinates coincide with those of radio sources from the list of IVS (International VLBI Service for Geodesy…
The space astrometry mission GAIA will construct a dense optical QSO-based celestial reference frame. For consistency between optical and radio positions, it will be important to align the GAIA frame and the International Celestial…
The space astrometry mission GAIA will construct a dense optical QSO-based celestial reference frame. For consistency between the optical and radio positions, it will be important to align the GAIA frame and the International Celestial…
We obtained optical spectra of four objects identified with variable radio sources. Three objects (0029+0554, 0400+0550, 2245+0500) were found to be quasars with redshifts of 1.314, 0.761, and 1.091. One object (2349+0534) has a continuum…
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…
The alignment between the radio-based International Celestial Reference Frame (ICRF) and the optical Gaia Celestial Reference Frame (Gaia-CRF) is critical for multi-waveband astronomy, yet systematic offsets at the optical bright end (G<13)…
We present optical spectroscopy secured at the 10m Gran Telescopio Canarias of the counterparts of 20 extragalactic gamma-ray sources detected by the Fermi satellite. The observations allow us to investigate the nature of these sources and…
A new list of physical characteristics of 3914 astrometric radio sources, including all 717 ICRF-Ext.2 sources, observed during IVS and NRAO VCS sessions have been compiled. The list includes source type, redshift and visual magnitude (if…
Since the early sixties, our view of radio galaxies and quasars has been drastically shaped by discoveries made thanks to observations of radio sources listed in the Third Cambridge catalog and its revised version (3CR). However, the…
We investigate a sample of 3412 {\it International Celestial Reference Frame} (ICRF3) extragalactic radio-loud sources with accurate positions determined by VLBI in the S/X band, mostly active galactic nuclei (AGN) and quasars, which are…
A new sample of very powerful radio galaxies is defined from the Molonglo Reference Catalogue, according to the criteria S (408 MHz) > 5 Jy, -30 < Dec < 10 degrees, |b| > 10 degrees. The sample is selected to have similar properties to the…
The fourth-DR3 version (4FGL-DR3) of the Fermi/LAT catalogue of $\gamma$-ray sources contains $\sim$ 1000 objects at a galactic latitude |b| > 10$^{\circ}$ which are not identified with an optical counterpart (UGS). We performed a…
We present a sample of 723 optically selected BL Lac candidates from the SDSS DR7 spectroscopic database encompassing 8250 deg^2 of sky; our sample constitutes one of the largest uniform BL Lac samples yet derived. Each BL Lac candidate has…
The International Celestial Reference Frame (ICRF, Ma et al. 1998) is currently the best realization of a quasi-inertial reference system. It is based on more than 10 years of cumulated geodetic and astrometric VLBI observations of compact…