623 research outputs found
Identification of Potential Weak Target Radio Quasars for ASTRO-G In-Beam Phase-Referencing
We apply an efficient selection method to identify potential weak Very Long
Baseline Interferometry (VLBI) target quasars simply using optical (SDSS) and
low-resolution radio (FIRST) catalogue data. Our search is restricted to within
12" from known compact radio sources that are detectable as phase-reference
calibrators for ASTRO-G at 8.4 GHz frequency. These calibrators have estimated
correlated flux density >20 mJy on the longest ground-space VLBI baselines. The
search radius corresponds to the primary beam size of the ASTRO-G antenna. We
show that ~20 quasars with at least mJy-level expected flux density can be
pre-selected as potential in-beam phase-reference targets for ASTRO-G at 8.4
GHz frequency. Most of them have never been imaged with VLBI. The sample of
these dominantly weak sources offers a good opportunity to study their radio
structures with unprecedented angular resolution provided by Space VLBI. The
method of in-beam phase-referencing is independent from the ability of the
orbiting radio telescope to do rapid position-switching manoeuvres between the
calibrators and the nearby reference sources, and less sensitive to the
satellite orbit determination uncertainties.Comment: 5 pages, accepted for the Publ. Astron. Soc. Japan (Vol. 61, No. 1,
Feb 2009
Cone Monotonicity: Structure Theorem, Properties, and Comparisons to Other Notions of Monotonicity
In search of a meaningful 2-dimensional analog to mono- tonicity, we
introduce two new definitions and give examples of and dis- cuss the
relationship between these definitions and others that we found in the
literature. Note: After we published the article in Abstract and Applied
Analysis and after we searched multiple times for previous work, we discovered
that Clarke at al. had introduced the definition of cone monotonicity and given
a characterization. See the addendum at the end of this paper for full
reference information
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
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