Effect of the reference radio source selection on nutation offset estimations

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Effect of the reference radio source selection on
nutation offset estimations

• J. Sokolova, Pulkovo Observatory RAS

The X Finnish-Russian Radio Astronomy Symposium
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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).

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ICRF
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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.  
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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)
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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)

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Inconsistencies between selection schemes
results

• Connection between ICRF list and M.
  Feissel-Vernier list

• Connection between ICRF list and P. Charlot list

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Example of inconsistenciesby some sources
Stable, - Unstable
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2201315

• 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

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2201315
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 15 Jul 2004
Radiomap on 13 Oct 1995
The time series of the 2201315 demonstrate the
synchronous change in both coordinates after
2001.0
Time series
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2145067

• 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 (a)
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2145067

• The set of radio images shows
• a long jet in the south-east
• direction
• structure of the jet is variable.

Radiomap on 21 Jun 1999
Radiomap on 09 Feb 1998
Time series (a)
The time series of the 2145067 right ascension
component show linear trend and quasi periodical
variations of 1.5 mas range
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4C39.25 (0923392)

• 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 (a)
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4C39.25 (0923392)
Radiomap on 16 Jun 2002
Radiomap on 15 Feb 2004
Radiomap on 01 May 2001
Time series (a)
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4C39.25 (0923392)
Radio Reference Frame Image Database (RRFID)
1985-1997 years.
1995-1998 years.
Time series (a)
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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.
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Impact of source instabilityon nutation time
series

• Radiosource 2201315

Nutation Dpsi offset difference
Time series (a, d)
Nutation Deps offset difference
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Impact of source instabilityon nutation time
series

• Radio source 2145067

Time series (a)
Nutation Deps offset difference
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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
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Thank you for your attention!