GaiaData Release 1

Abstract

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.

LQAC-4: Fourth release of the Large Quasar Astrometric Catalogue

“…9, which shows the histograms of the distribution of the differences in right ascension and declination between the ICRF2 and the GDR1 catalogues. We can observe that the majority of values range in the interval ±5 mas in both histograms, which are similar to those presented in a more detailed study by Mignard et al (2016). We notice that ICRF2 positions are determined from VLBI radio observations, and that the link between the VLBI radiocentres and the Gaia photocentres was subject to recent deep investigations (Kovalev et al 2017;Petrov & Kovalev 2017a,b).…”

The Host Galaxy and Redshift of the Repeating Fast Radio Burst FRB 121102

“…The positional uncertainties in each axis are the quadratic sum of the astrometric tie against Gaia (of the order of 2 mas) and the centroid uncertainty on the image (between 20 and 50 mas). The Gaia frame is tied to the ICRF defined via radio VLBI to a ∼1 mas precision (Mignard et al 2016), much smaller than the centroid uncertainty. We find that the position of the persistent radio source seen with the EVN at an observing frequency of 5 GHz with a 1 mas precision (Marcote et al 2017) is offset from the galaxy centroids by 186±68 and 163±32 mas in the linedominated r′ and i′ images, and 286±64 mas in the continuum-dominated z′ image.…”

GaiaData Release 1

“…The detailed description of the production of the astrometric solution, as well as a more detailed statistical summary of the astrometry contained in Gaia DR1 can be found in Lindegren et al (2016). An in-depth discussion of the Gaia DR1 reference frame and the optical properties of ICRF sources is presented in Mignard et al (2016). 2.…”