133 research outputs found
High Precision Astrometric Millimeter VLBI Using a New Method for Atmospheric Calibration
We describe a new method which achieves high precision Very Long Baseline
Interferometry (VLBI) astrometry in observations at millimeter wavelengths. It
combines fast frequency-switching observations, to correct for the dominant
non-dispersive tropospheric fluctuations, with slow source-switching
observations, for the remaining ionospheric dispersive terms. We call this
method Source-Frequency Phase Referencing. Provided that the switching cycles
match the properties of the propagation media, one can recover the source
astrometry. We present an analytic description of the two-step calibration
strategy, along with an error analysis to characterize its performance. Also,
we provide observational demonstrations of a successful application with
observations using the Very Long Baseline Array at 86 GHz of the pairs of
sources 3C274 & 3C273 and 1308+326 & 1308+328, under various conditions. We
conclude that this method is widely applicable to millimeter VLBI observations
of many target sources, and unique in providing bona-fide astrometrically
registered images and high precision relative astrometric measurements in
mm-VLBI using existing and newly built instruments.Comment: Astronomical Journal, accepted for publicatio
Multi-step VLBI observations of weak extragalactic radio sources to align the ICRF and the future GAIA frame
The space astrometry mission GAIA will construct a dense optical QSO-based
celestial reference frame. For consistency between optical and radio positions,
it will be important to align the GAIA frame and the International Celestial
Reference Frame (ICRF) with the highest accuracy. Currently, it is found that
only 10% of the ICRF sources are suitable to establish this link, either
because they are not bright enough at optical wavelengths or because they have
significant extended radio emission which precludes reaching the highest
astrometric accuracy. In order to improve the situation, we have initiated a
VLBI survey dedicated to finding additional suitable radio sources for aligning
the two frames. The sample consists of about 450 sources, typically 20 times
weaker than the current ICRF sources (down to the 20 mJy flux level), which
have been selected by cross-correlating optical and radio catalogues. This
paper presents the observing strategy to detect, image, and measure accurate
positions for these sources. It will also provide results about the VLBI
detectability of the sources, as derived from initial observations with the
European VLBI Network in June and October 2007. Based on these observations, an
excellent detection rate of 89% is found, which is very promising for the
continuation of this project
Changes in the measured image separation of the gravitational lens system, PKS 1830-211
We present eight epochs of 43 GHz, dual-polarisation VLBA observations of the
gravitational lens system PKS 1830-211, made over fourteen weeks. A bright,
compact ``core'' and a faint extended ``jet'' are clearly seen in maps of both
lensed images at all eight epochs.
The relative separation of the radio centroid of the cores (as measured on
the sky) changes by up to 87 micro arcsec between subsequent epochs.
A comparison with the previous 43 GHz VLBA observations (Garrett et al. 1997)
made 8 months earlier show even larger deviations in the separation of up to
201 micro arcsec. The measured changes are most likely produced by changes in
the brightness distribution of the background source, enhanced by the
magnification of the lens. A relative magnification matrix that is applicable
on the milliarcsecond scale has been determined by relating two vectors (the
``core-jet'' separations and the offsets of the polarised and total intensity
emission) in the two lensed images. The determinant of this matrix,
-1.13 +/-0.61, is in good agreement with the measured flux density ratio of
the two images. The matrix predicts that the 10 mas long jet, that is clearly
seen in previous 15 and 8.4 GHz VLBA observations (Garrett et al. 1997,
Guirado et al. 1999), should correspond to a 4 mas long jet trailing to the
south-east of the SW image. The clear non-detection of this trailing jet is a
strong evidence for sub-structure in the lens and may require more realistic
lens models to be invoked, e.g. Nair & Garrett (2000).Comment: 8 pages, 5 figure
VLBI observations of weak extragalactic radio sources for the alignment of the future GAIA frame with the ICRF
The space astrometry mission GAIA will construct a dense optical QSO-based
celestial reference frame. For consistency between the optical and radio
positions, it will be important to align the GAIA frame and the International
Celestial Reference Frame (ICRF) with the highest accuracy. Currently, it is
found that only 10% of the ICRF sources are suitable to establish this link,
either because they are not bright enough at optical wavelengths or because
they have significant extended radio emission which precludes reaching the
highest astrometric accuracy. In order to improve the situation, we have
initiated a VLBI survey dedicated to finding additional high-quality radio
sources for aligning the two frames. The sample consists of about 450 sources,
typically 20 times weaker than the current ICRF sources, which have been
selected by cross-correlating optical and radio catalogues. This paper presents
the observing strategy and includes preliminary results of observation of 224
of these sources with the European VLBI Network in June 2007
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