Quasars can be used to verify the parallax zero-point of theTycho-GaiaAstrometric Solution

Abstract

Context. The Gaia project will determine positions, proper motions, and parallaxes for more than one billion stars in our Galaxy. It is known that Gaia's two telescopes are affected by a small but significant variation of the basic angle between them. Unless this variation is taken into account during data processing, e.g. using on-board metrology, it causes systematic errors in the astrometric parameters, in particular a shift in the parallax zero-point. Previously, we suggested an early reduction of Gaia data for the subset of Tycho-2 stars (Tycho-Gaia Astrometric Solution; TGAS). Aims. We investigate whether quasars can be used to independently verify the parallax zero-point in early data reductions. This is not trivially possible as the observation interval is too short to disentangle parallax and proper motion for the quasar subset. Methods. We repeat TGAS simulations but additionally include simulated Gaia observations of quasars from ground-based surveys. All observations are simulated with basic angle variations. To obtain a full astrometric solution for the quasars in TGAS we explore the use of prior information for their proper motions. Results. It is possible to determine the parallax zero-point for the quasars with a few µas uncertainty, and it agrees to a similar precision with the zero-point for the Tycho-2 stars. The proposed strategy is robust even for quasars exhibiting significant spurious proper motion due to a variable source structure, or when the quasar subset is contaminated with stars misidentified as quasars. Conclusions. Using prior information about quasar proper motions we could provide an independent verification of the parallax zero-point in early solutions based on less than one year of Gaia data.

GaiaData Release 1

“…One possible exception is the constant part of the cos Ω coefficient, corresponding to C 1,0 in Eq. (A.11), which is almost completely degenerate with respect to a global error of the parallax zero point (Lindegren et al 1992;Michalik & Lindegren 2016). Further details will be discussed elsewhere.…”

“…4). By assuming that these sources have negligible proper motions (the prior was set to 0 ± 0.01 mas yr −1 in each component) it was possible to solve the positions and parallaxes for most of them as described by Michalik & Lindegren (2016). At the end of phase E these objects had positions and parallaxes with median (inflated) standard uncertainties of about 1 mas.…”

“…The heliotropic spin phase Ω increases by 360 • for each ∼6 h spin period and is zero when the direction to the apparent Sun is symmetrically located between the two fields of view (see Fig. 1 in Michalik & Lindegren 2016). Figure A.2 shows an example of the line-of-sight variations in the preceding field of view during a one-day interval (four successive spin periods).…”

Analysis of the Bayesian Cramér-Rao lower bound in astrometry

“…In this case, the prior information is derived from reasonable assumptions regarding the distributions of proper motions and parallaxes. On the other hand, in (Michalik & Lindegren 2016) they show how the prior information regarding QSO proper motions could provide an independent verification of the parallax zero-point for the early reduction of Gaia data in the context of the HTPM project, which uses Hipparcos stars as first epoch (Mignard 2009;Michalik et al 2014), or the TGAS project, which uses the Tycho-2 stars as first epoch (Michalik et al 2015a). …”

“…The use of a Bayesian approach to astrometry is not new; we note, for example, that recently (Michalik et al 2015b) and (Michalik & Lindegren 2016) have proposed analyzing part of the data from the Gaia astrometric satellite using a suitable chosen set of priors in a Bayesian context. In Michalik et al (2015b), the feasibility of the Bayesian approach, especially for the analysis of stars with poor observation histories, is demonstrated through global astrometric solutions of simulated Gaia observations.…”