Plans for an accurate alignment of the
VLBI frame and the future Gaia frame
G Bourda
B d & P.
P Charlot
Ch l
G.
Laboratoire d’Astrophysique de Bordeaux
France
Journées Systèmes de Référence Temps-Espace
19-21 September 2011, Vienna − Austria
Outline
Plans for an accurate alignment of the VLBI frame and the future Gaia frame
y Context
y Aligning
g g VLBI & Gaia frames
y ICRF2 suitability
y VLBI observational plans
y Beyond the alignment
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Journées Systèmes de Référence Temps-Espace
19-21 September 2011, Vienna − Austria
Context
B 2020:
By
2020 T
Two extragalactic
t
l ti celestial
l ti l reference
f
frames
f
available
il bl
VLBI (Radio)
Position accuracy:
1997: ICRF1 − 717 sources − σ ≥ 250 μas
2009: ICRF2 − 3414 sources − σ ≥ 60 μas
2015-2020: ICRF3 ???
Gaia (Optical magnitude ≤ 20)
Anticipated position accuracy:
20 000 QSOs @ V≤18 Æ 16 μas ≤ σ ≤ 70 μas
500 000 QSOs @ V≤20 Æ 16 μas ≤ σ ≤ 200 μas
Lindegren et al. 2008
Linking these two frames is important:
• to ensure continuity of the fundamental celestial reference frame
• to register optical & radio positions with the highest accuracy
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Journées Systèmes de Référence Temps-Espace
19-21 September 2011, Vienna − Austria
Context
Issues in the VLBI/Gaia frames alignment
y Optical variability
y Optical structure
y Core shifts
Credit: A. Lobanov
Æ Invest in source qqualityy (both
(
radio and optical)
p
)
Æ Needs sample larger than actually required because
sources likely to be rejected at later stages
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Journées Systèmes de Référence Temps-Espace
some
19-21 September 2011, Vienna − Austria
Aligning VLBI & Gaia frames (1)
y Requirements:
3 Several hundreds of common sources
3 With a uniform sky coverage
3 Strong enough in the radio to be detectable with VLBI arrays
3 Common sources must have:
Accurate Gaia position
Æ Optically-bright (mag. ≤ 18; Mignard 2003)
Accurate VLBI pposition Æ Good astrometric qqualityy ((no extended
VLBI structures)
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Journées Systèmes de Référence Temps-Espace
19-21 September 2011, Vienna − Austria
Aligning VLBI & Gaia frames (2)
y Current status:
3 70 suitable sources found in ICRF (Bourda et al.
al 2008)
3 VLBI observations of 398 candidate sources in progress (Bourda et al. 2010)
Æ ~200 additional sources?
9 ICRF2 examined for suitable sources
Æ ICRF2 optical couterparts within LQAC?
Æ How many with a proper structure index?
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Journées Systèmes de Référence Temps-Espace
LQAC = Large Quasar Astrometric
C t l
Catalog;
S h ett al.
Souchay
l 2009
19-21 September 2011, Vienna − Austria
How many suitable sources in the ICRF2?
¾ 1128 sources with a proper optical couterpart for the alignment
(V or R or I < 18 from LQAC)
¾ Of these 201 have an appropriate structure index (i.e. < 3.0)
(not all 1128 sources have a calculated structure index)
SI=3.0
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Journées Systèmes de Référence Temps-Espace
19-21 September 2011, Vienna − Austria
Flux density distribution
201 current ICRF2 suitable sources
Skyy distribution
201 current ICRF2 suitable sources
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Journées Systèmes de Référence Temps-Espace
19-21 September 2011, Vienna − Austria
VLBI observational plans
y Finalize source sample for the alignment
y Measure source positions before Gaia launch (for those not yet
well known)
y Monitor sources during the Gaia mission
¾ Control VLBI position stability and accuracy
¾ Make VLBI images to control structure
¾ Use Gaia scanning
g law to pplan the VLBI observations
y Observing networks
¾ IVS for the stronger sources
¾ EVN/VLBA/DSN for the weaker sources
¾ High frequency observations to reduce core-shifts?
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Journées Systèmes de Référence Temps-Espace
19-21 September 2011, Vienna − Austria
Beyond the alignment …
y One will be able to compare the VLBI and Gaia positions to
within their error bars (< 100 µas) for any detected celestial
object
y Measure relative location of radio and optical emission
¾ Probe
P b AGN geometry
¾ … but also for stellar objects
10 Journées Systèmes de Référence Temps-Espace
19-21 September 2011, Vienna − Austria
Thank you for your attention ...