Abstract
Aims. The IVS Working Group on Galactic Aberration (WG8) was established to investigate issues related to incorporating the effect of Galactic aberration in IVS analysis. The circular motion of the solar system barycenter around the Galactic center causes a change in aberration, which in the case of geodetic VLBI observing is over time scales of several decades. One of the specific goals was to recommend a Galactic aberration model to be applied by the IAU ICRF3 working group in the generation of ICRF3 as well as in other IVS analysis. Studies made by working group members have shown that the three-dimensional acceleration vector of the solar system barycenter can be estimated from VLBI delay observations. Methods. Among the working group members, three methods were used to estimate the acceleration vector. One is to directly estimate the acceleration vector as a global parameter. The second is to estimate the acceleration vector from source proper motions determined from estimated source position time series. A third method estimated a global reference frame scale parameter for each source and derived the acceleration vector from these estimates. The acceleration vector estimate consists of a galactocentric component along with the non-galactocentric components. Results. The geodetic reference frame VLBI estimates of the galactocentric aberration constant from the different working group members are in the range 5.1–6.4 μas yr−1. These estimates are relatively close to independent estimates of 4.8–5.4 μas yr−1 that can be derived from astrometric measurements of proper motions and parallaxes of masers in the Milky Way galaxy. Based on the most recent geodetic VLBI solutions, we find an upper bound of 0.8 μas yr−1 for the non-galactocentric component of the secular aberration. Conclusions. The working group made a recommendation only for the galactocentric component of the observed acceleration vector. For the recommended galactocentric aberration constant, the working group chose a geodetic value to be consistent with geodetic VLBI applications. The recommended value 5.8 μas yr−1 was estimated directly in a global solution that used the ICRF3 solution data set: 1979–May 2018.
Highlights
The International VLBI Service for Geodesy and Astrometry (IVS) Working Group on Galactic Aberration (WG8) was established by the IVS Directing Board at its meeting in November 2015 with the work to begin in 2016
Since none of the solutions reported in the upper section of Table 2 used all of the available data, a new Calc/Solve global solution was run using all of the data that was to be used for the ICRF3 solution with a reference epoch of 2015.0
The reasons for this choice are (1) it is close to the current time so the effect aberration is small for current applications that do not have the ability to model the effect and (2) it is close to the Gaia DR2 reference epoch of 2015.5 so that even though Gaia does not yet model Galactic aberration, one can compare VLBI and Gaia source positions without much error since the VLBI solution models the decades long effect of aberration
Summary
The International VLBI Service for Geodesy and Astrometry (IVS) Working Group on Galactic Aberration (WG8) was established by the IVS Directing Board at its meeting in November 2015 with the work to begin in 2016. Because of the extreme length of the Galactic rotation period, it has been standard practice to absorb this large, nearly constant effect into the reported source positions Based on this investigation, WG8 was tasked to formulate a recommendation for an aberration correction to be applied in IVS data analysis and to be provided to the ICRF3 working group (Jacobs et al 2012). We consider the effects on estimated source positions that result when an aberration model is applied in a VLBI CRF solution. 4 and 5, we consider possible choices of the model aberration constant: (1) a value determined from recent parallax and proper motion measurements of Galactic masers, or (2) a geodetic VLBI determined value. We discuss placing an upper bound on non-Galactic center acceleration of the SSB based on our estimates of the secular aberration drift vector
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