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Effect of the reference radio source selection on nutation offset estimations

Effect of the reference radio source selection on nutation offset estimations. J. Sokolova, Pulkovo Observatory RAS. The X Finnish-Russian Radio Astronomy Symposium. ICRF.

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Effect of the reference radio source selection on nutation offset estimations

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  1. Effect of the reference radio source selection on nutation offset estimations • J. Sokolova, Pulkovo Observatory RAS The X Finnish-Russian Radio Astronomy Symposium

  2. ICRF • The first realization of the ICRF (catalog) was constructed in 1995 by a reanalysis of all VLBI observations 1979-1995 (Ma et al. 1998). It contains coordinates of 608 sources. • The objects are divided in three subsets: “defining”, “candidate” and “other”. • 212 of these are “defining” sources. They are a core of the ICRF. • 294 “candidate” sources. • 102 ‘other’ sources. • Two extensions have been announced since 1995 covering observations at 1994 through 2002 (Fey et al. 2001, 2004).

  3. ICRF

  4. Toward the ICRF-2 The XXVI-th General Assembly of IAU formed a working group for a purpose to oversee the generation of the second realization of the ICRF from VLBI observations of extragalactic radio sources. One of the primary goals is the selection of defining sources and the mitigation of source position variations to improve the stability of the ICRF.

  5. Toward the ICRF-2 The problem of stable radio source selection is now one of the key problems for ICRF improvement (MacMillan and Ma 2007, Tornatore and Charlot 2007). Only two subsets of ICRF sources : “stable”, “unstable” (according the decision of ICRF-2 WG)

  6. Methods of selection Four stability criteria based on different schemes have been used for selection of reference radio sources: • The list of the ICRF defining sources, published by Ma et.al. (1998) • The list of stable radio sources determined by M.Feissel-Vernier (2003) • The list of structure indices of radio sources set up by P.Charlot and A. Fey (1997) • The list of G. Engelhardt and V. Thorandt (2006)

  7. Inconsistencies between selection schemes results Connection between ICRF list and M. Feissel-Vernier list Connection between ICRF list and P. Charlot list

  8. Example of inconsistenciesby some sources “ + ” Stable, “ - ” Unstable

  9. 2201+315 Unstable in ICRF (“other” group) Unstable by Patrick Charlot (Index 3 by X-Band) • Stable source by M. Feissel-Vernier • Stable by G.Engelhardt and V. Thorandt

  10. 2201+315 The centroid of radio brightness had moved on 500 µas from its original position by the end of 2003 and returned back by the end of 2004. The positional variations can be explained by jet motion in the south-west direction between 2001 and 2004. The direction is confirmed by the X-Band radio images. Radiomap on 13 Oct 1995 Radiomap on 15 Jul 2004 The time series of the 2201+315 demonstrate the synchronous change in both coordinates after 2001.0 Time series

  11. 2145+067 • Stable in ICRF (“defining” group) • Stable source by M. Feissel-Vernier • Stable by Patrick Charlot (Index 2 by X-Band) Unstable by Engelhardt and V. Thorandt Time series (α)

  12. 2145+067 • The set of radio images shows • a long jet in the south-east • direction • structure of the jet is variable. Radiomap on 09 Feb 1998 Radiomap on 21 Jun 1999 Time series (α) The time series of the 2145+067 right ascension component show linear trend and quasi periodical variations of 1.5 mas range

  13. 4C39.25 (0923+392) Unstable in ICRF (“other” group) Unstable source by M. Feissel-Vernier Unstable by Engelhardt and V. Thorandt • Stable by P. Charlot (Index 2 by X-Band) Time series (α)

  14. 4C39.25 (0923+392) Radiomap on 16 Jun 2002 Radiomap on 15 Feb 2004 Radiomap on 01 May 2001 Time series (α)

  15. 4C39.25 (0923+392) Radio Reference Frame Image Database (RRFID) 1985-1997 years. 1995-1998 years. Time series (α)

  16. Impact of source instabilityon nutation time series Solutions: To study the impact of source instability on nutation offset estimations, several nutation time series have been calculated during the daily solution (Kalman filter) in different modes. 1. Coordinates of all ICRF sources are fixed 2. Observations of analyzable source are excluded from processing The difference between these two solutions allows us to treate an impact of analyzable source on nutation offset estimation.

  17. Impact of source instabilityon nutation time series • Radiosource 2201+315 Nutation Dpsi offset difference Time series (α, δ) Nutation Deps offset difference

  18. Impact of source instabilityon nutation time series • Radio source 2145+067 Time series (α) Nutation Deps offset difference

  19. Summary • All four selection schemes have considerable inconsistencies. • Variations of radio sources coordinates significant affect on nutation time series. • Work on radio source coordinates variations is being continued. Radio Reference Frame Image Database (RRFID) of the United States Naval Observatory (USNO) was used in this research

  20. Thank you for your attention!

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