418 research outputs found
A Review of VLBI Instrumentation
The history of VLBI is summarized with emphasis on the technical aspects. A
summary of VLBI systems which are in use is given, and an outlook to the future
of VLBI instrumentation.Comment: 8 pages. No figures. Proceedings of the 7th European VLBI Network
Symposium held in Toledo, Spain on October 12-15, 2004. Editors: R.
Bachiller, F. Colomer, J.-F. Desmurs, P. de Vicente (Observatorio Astronomico
Nacional), p. 237-244. Needs evn2004.cl
Solving the polarization problem in ALMA-VLBI observations
The Atacama Large mm-submm Array (ALMA) is, by far, the most sensitive
mm/submm telescope in the World. The ALMA Phasing Project (APP) will allow us
to phase-up all the ALMA antennas and use them as one single VLBI station. This
will be a key component of the Event Horizon Telescope (EHT), a Global VLBI
array at millimeter wavelengths. A problem in the APP is the calibration and
conversion of the polarization channels. Most VLBI stations record their
signals in a circular basis, but the ALMA receivers record in a linear basis.
The strategy that will be followed in the phased-ALMA VLBI observations will be
to correlate in "mixed" basis (i.e., linear versus circular) and convert the
visibilities to a pure circular basis after the correlation. We have developed
an algorithm to perform such a polarization conversion of the VLBI
visibilities. In these proceedings, we present the basics of the PolConvert
algorithm and discuss on the polarization conversion in the general case were
single dishes (besides phased arrays) record with linear receivers in VLBI
observations. We show some results of PolConvert applied to realistic
simulations, as well as a test with real VLBI observations at 86\,GHz between
the Onsala radiotelescope (recording in linear basis) and the Effelsberg
radiotelescope (recording in circular basis).Comment: To appear in the Proceedings of the 12th European VLBI Network
Symposium (7-10 Oct 2014, Cagliary, Italy
Searching for low mass objects around nearby dMe radio stars
Nearby M-dwarfs are best suited for searches of low mass companions. VLBI
phase-referencing observations with sensitive telescopes are able to detect
radio star flux-densities of tenths of mJy as well as to position the star on
the sky with submilliarcsecond precision. We have initiated a long-term
observational program, using EVN telescopes in combination with NASA DSN
dishes, to revisit the kinematics of nearby, single M dwarfs. The precision of
the astrometry allows us to search for possible companions with masses down to
1 Jupiter mass. In this contribution we report preliminary results of the first
observation epochs, in which we could detect some of the radio stars included
in our program.Comment: Proceedings of the 6th European VLBI Network Symposium, Ros E.,
Porcas R.W., Lobanov A.P., & Zensus J.A. (eds.), MPIfR, Bonn, Germany, p.
255-258 (2002). 4 pages, 3 figures, needs evn2002.cl
Spectral Properties of the Core and the VLBI-Jets of Cygnus A
We present a detailed VLBI study of the spectral properties of the inner core
region of the radio galaxy Cygnus A at 5 GHz, 15 GHz, 22 GHz, 43 GHz and 86
GHz. Our observations include an epoch using phase-referencing at 15 GHz and 22
GHz and the first successful VLBI observations of Cygnus A at 86 GHz. We find a
pronounced two-sided jet structure, with a steep spectrum along the jet and an
inverted spectrum towards the counter-jet. The inverted spectrum and the
frequency-dependent jet-to-counter-jet ratio suggest that the inner counter-jet
is covered by a circum-nuclear absorber as it is proposed by the unified
scheme.Comment: 2 pages, 2 figures, Proceedings of the 7th EVN Symposium held in
Toledo, Spain in October 2004, needs evn2004.cl
Compact Structure in FIRST Survey Sources
We present preliminary results from a statistical survey of compact structure
in faint radio sources. Around 1000 sources from the VLA FIRST survey (flux
densities larger than 1 mJy at 1.4 GHz) have been observed with the
single-baseline interferometer Effelsberg-Arecibo. We observed each source,
selected from a narrow strip of sky at declination 28 deg, for just one minute.
The baseline sensitivity at 1.4 GHz, using 512 Mb/s recording, is such that any
FIRST source, selected at random, would be detected if most of its flux density
is in compact structure. We discuss the detection-rate statistics from one
epoch of these observations.Comment: 4 pages. 12 figures. Proceedings of the 7th European VLBI Network
Symposium held in Toledo, Spain on October 12-15, 2004. Editors: R.
Bachiller, F. Colomer, J.-F. Desmurs, P. de Vicente (Observatorio Astronomico
Nacional), p. 31-34. Needs evn2004.cl
Proper motion in Cygnus A
Our recent VLBI observations of the prominent FR II radio galaxy Cygnus A
with the EVN and the VLBA reveal a pronounced two-sided jet structure. At 5
GHz, we now have 4 epochs from 1986, 1991 (Carilli et al., 1991 & 1994), 1996
and 2002 from which we could derive the kinematics of the jet and counter-jet.
On the jet side and on mas scales, the jet seems to accelerate from (Krichbaum et al. 1998) at core-separations near 1 mas to
at mas ( km s
Mpc, ). For the first time we also measure significant
structural variability on the counter-jet side. For this, we derive a motion of
at mas. The flat spectrum of the inner
region of the counter-jet (free-free absorption) and the frequency dependence
of the jet to counter-jet ratio support strong evidence for an obscuring torus
in front of the counter-jet (Bach et al. in prep, Krichbaum et al. 1998).Comment: 4 pages, 6 figures, appears in: Proceedings of the 6th European VLBI
Network Symposium held on June 25th-28th in Bonn, Germany. Edited by: E. Ros,
R.W. Porcas, A.P. Lobanov, and J.A. Zensu
DiFX2: A more flexible, efficient, robust and powerful software correlator
Software correlation, where a correlation algorithm written in a high-level
language such as C++ is run on commodity computer hardware, has become
increasingly attractive for small to medium sized and/or bandwidth constrained
radio interferometers. In particular, many long baseline arrays (which
typically have fewer than 20 elements and are restricted in observing bandwidth
by costly recording hardware and media) have utilized software correlators for
rapid, cost-effective correlator upgrades to allow compatibility with new,
wider bandwidth recording systems and improve correlator flexibility. The DiFX
correlator, made publicly available in 2007, has been a popular choice in such
upgrades and is now used for production correlation by a number of
observatories and research groups worldwide. Here we describe the evolution in
the capabilities of the DiFX correlator over the past three years, including a
number of new capabilities, substantial performance improvements, and a large
amount of supporting infrastructure to ease use of the code. New capabilities
include the ability to correlate a large number of phase centers in a single
correlation pass, the extraction of phase calibration tones, correlation of
disparate but overlapping sub-bands, the production of rapidly sampled
filterbank and kurtosis data at minimal cost, and many more. The latest version
of the code is at least 15% faster than the original, and in certain situations
many times this value. Finally, we also present detailed test results
validating the correctness of the new code.Comment: 28 pages, 9 figures, accepted for publication in PAS
Magnetic Field Geometry of the Broad Line Radio Galaxy 3C111
Very Long Baseline Polarimetric observations of the Broad Line Radio galaxy
3C111 performed in July and September of 1996 at 8 and 43 GHz reveal rapidly
evolving parsec-scale radio structure after a large millimetre outburst. The
B-field geometry is not simple. We present a first analysis of possible Faraday
and optical depth effects based on a comparison of the polarization images for
the two frequencies.Comment: 4 pages, 4 figures, Proceedings of conference "The Physics of
Relativistic Jets in the Chandra and XMM Era", eds. G. Brunetti, D.E. Harris,
R.M. Sambruna, and G. Setti, submitted to New Astronomy Revie
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