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Linking the ICRF and the future Gaia optical frame

Linking the ICRF and the future Gaia optical frame. G. Bourda, P. Charlot , A. Collioud Laboratoire d’Astrophysique de Bordeaux R. Porcas Max Planck Institut fur Radioastronomie S. Garrington Jodrell Bank Observatory. The Gaia astrometric mission.

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Linking the ICRF and the future Gaia optical frame

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  1. Linking the ICRF and the future Gaia optical frame G. Bourda, P. Charlot, A. Collioud Laboratoire d’Astrophysique de Bordeaux R. Porcas Max Planck Institut fur Radioastronomie S. Garrington Jodrell Bank Observatory

  2. The Gaia astrometric mission • Gaia will observe 1 billion stars and 500 000 QSOs • Astrometric accuracy • Launch: 2012 • Preliminarycatalog: ~ 2015 • Final catalog: 2018-2020 VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  3. Stellar physics Structure galactique Stellar Systems Quasars & galaxies Exo-planets Solar system Fundamental physics Reference frame Which science with Gaia? VLBA Astrometry Workshop, Socorro, 21-23 July 2009 M2, Paris - 08/01/2009 F. Mignard

  4. With a flawless selection : GAIA will observe on ~ half of the sky • B  16 200 • B  18 20 000 • B  20 400 000 Hartwick & Schade, 1990 Reference frame VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  5. 19< G < 20 G < 18 18< G < 19 Simulations of Gaia catalog (Slezak and Mignard) Galactic absorption included in the visible range Automatic recognition of QSOs based on photometry VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  6. Context in 5-10 years ICRF By 2015-2020, two extragalactic celestial reference frames will be available Gaia Position accuracy: ICRF: σ = 250 µas ICRF-2: σ = 40 µas Position accuracy: 16 µas ≤ σ ≤ 70 µas @ 15 ≤ V ≤ 18 • Linking the two frames is important • to ensure continuity of the reference frame • to register optical and radio positions with the highest accuracy Link sources must have: accurate VLBI positions  no structure accurate Gaia positions  V≤ 18 VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  7. Currentstatus of the link (1) Optical magnitude of ICRF sources Astrometric source quality ~30% ICRF ~10% ICRF VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  8. Currentstatus of the link (2) Astrometric source quality (SI=structure index) • Distribution of SI worse for the V<18 sources than for the 18 < V < 20 sources(Bourda et al. 2008) • Confirmed by comparison of ICRF position accuracies for the two categories The potentially best GAIA sources for the alignmentwith the ICRF are not the best ICRF sources !!! • Only 10% of the current ICRF sources suitable  Must find new candidates VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  9. Overview of observing program • Sampleconsists of 447 sources • Targetsselectedfrom NVSS, excluding ICRF and VCS sources • Optical magnitude V ≤ 18 • Total flux density (NVSS) ≥ 20 mJy • δ≥ -10º • ObservingStrategy • VLBI detection • Imaging • Accurateastrometry (for the most compact sources) VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  10. Step 1: VLBI detection • Two 48-hr EVN experiments (S/X geodetic style @ 1 Gbps) • 224 sources (mostlyfrom CLASS) observed in June 2007 (project EC025A) • 223 sources observed in October 2007 (project EC025B) • 4 or 5-station network • Effelsberg (100m), Medicina (32m), Noto (32m), Onsala (25m) • + Robledo (70m) for EC025B • S and X detection rates • EC025A: 96% • EC025B: 82% Overall detection rate: ~ 89 % (398 sources) VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  11. Flux density distribution (1) All flux densities are the meancorrelated flux densities over all baselines X-band (mJy) S-band (mJy) VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  12. Flux density distribution (2) VCS: Median = 210 mJy Comparison of X-band flux density distribution in ICRF, VCS and for thisproject ICRF: Median = 700 mJy This project: Median = 26 mJy VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  13. Spectral index distribution median = -0.34 224 CLASS sources α VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  14. Step 2: imaging • 105 sources observed with global VLBI (VLBA + EVN) in March 2008 (selected from EC025A) • 48-hour dual-frequency S/X @ 512 Mbps • Schedule optimized for imaging • Results • All 105 sources successfully imaged at both X + S bands • Dynamic range: ~ 1% VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  15. X-band images (1) Some very good link sources VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  16. X-band images (2) Some not so good link sources… VLBA Astrometry Workshop, Socorro, 21-23 July 2009

  17. Prospects • Image the remaining 293 targets in oursample • Carry out global astrometry on the most compact sources and gettheir position to betterthan <100 μas • Search for more candidates in the ICRF-2/VCS lists • Attack the southern hemisphere • Issues of core shifts • Task now officially recognized as part of the Gaia DPAC (Data Processing Analysis Consortium) • Ultimately the Gaia link sources should form the basis of ICRF-3 by 2015 VLBA Astrometry Workshop, Socorro, 21-23 July 2009

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