About IVS

Working Groups: IVS Working Group on Galactic Aberration

Purpose. This new IVS Working Group (WG) will investigate the issues related to incorporating the effect of galactic aberration in IVS analysis. The effect is not negligible in terms of future microarcsecond astrometry. The WG will investigate: 1) what value of secular aberration to apply in an a priori model of aberration, and 2) formulate a recommendation to the ICRF3 working group.

Justification. The effect of aberration is to cause apparent source positions to change over time. Several studies in recent years have shown that aberration can be estimated from VLBI data. The estimates are in the range 5–7 µas/yr and are close to independent estimates derived from parallax measurements of 5–5.5 µas/yr. Although the effect is small it is not negligible in terms of future µas astrometry. The aberration correction to radio source positions at J2000 is as large as 20–30 µas depending on the source coordinates and the median epoch of the observations of a source. The systematic drift due to an aberration drift of 5 µas/yr would lead to a dipole systematic error of 100 µas after 20 years.

Mandate. The primary objective of the WG is to develop a recommended aberration correction to be applied in VLBI analysis. Other issues include whether there are components of the aberration acceleration vector not directed towards the galactic center. An underlying issue is that applying apparent proper motion corrections due to aberration in VLBI analysis could require a redefinition of the ICRS. Such a redefinition is not something that the IVS cannot do but would have to be made by the IAU.

• Task 1: Value of the secular aberration drift constant to be applied.

  Initially the WG will investigate whether the application of an aberration model will result in an improvement of VLBI analysis. Measures of improvement include a better fit to data or better estimates of Earth orientation. The WG will also consider significance of the dipole systematic due to aberration compared to CRF noise floor, which in the case of ICRF2 was 40 µas.

  The WG will determine the level of accuracy required to meet future astrometric requirements. Possible choices of the constant are 1) a VLBI determined value, 2) a value determined by recent parallax measurements, 3) an average over different techniques. Application of the correction will require a specific epoch for source positions as well as the apparent proper motion for each source.

• Task 2: Direction of the aberration acceleration vector.

  Estimates of the aberration vector by different VLBI groups have significant components not directed toward the galactic center at the 20–30% level. The WG will investigate whether there are errors in the VLBI analysis that causes this or whether these components correspond to other physical causes of aberration.

• Task 3: Redefinition of the ICRS to account for aberration.

  The ICRF is defined by the positions of a set of defining sources that were assumed to have no measurable proper motion. The WG will study whether it is necessary to redefine the ICRS or whether we can simply apply an aberration proper motion correction in VLBI analysis similar to the way precession or annual aberration is accounted for during VLBI processing.

  Applying an aberration proper motion model opens the door to all causes of proper motion. In particular, estimation of apparent linear proper motions of all sources in a TRF/CRF solution yields a large range of linear proper motions, many of which are significantly larger than proper motion due to galactic aberration. Source position time series solutions indicate that apparent proper motion for many sources is nonlinear. The WG will investigate how to optimally handle these apparent proper motion estimates in the generation of an ICRF.

  The WG will provide the ICRF3 WG with our recommendation for an aberration correction.

Desired Outcomes:

1. Memos and other publications regarding the WG investigation.
2. A model of galactic aberration to be applied in VLBI analysis.
3. A recommendation and justification for a WG a priori model to be applied in the ICRF3 solution.

Working Group Members.
Dan MacMillan (NASA), Chair
John Gipson (NASA), Ex-officio
Chopo Ma (IERS Representative, NASA)
David Gordon (NASA)
Chris Jacobs (JPL)
Zinovy Malkin (Pulkovo Observatory)
Sebastien Lambert (Paris Observatory)
Oleg Titov (Geosciences Australia)
Minghui Xu (Shanghai Astronomical Observatory)
Alan Fey (USNO)
Norbert Zacarias (USNO)
Hana Krásná (Vienna Technical University)
Guangli Wang (SHAO)

Correspondents.
Ralph Gaume (NSF)
Valeri Makarov (USNO)

Reports. The final report of WG8 was accepted at the March 21, 2019 Directing Board meeting and the Working Group was closed. The final report was published as special report in the IVS 2017+2018 Biennial Report.