432 research outputs found
From Reference Frames to Relativistic Experiments: Absolute and Relative Radio Astrometry
Reference systems and frames are crucial for high precision absolute
astrometric work, and their foundations must be well-defined. The current
frame, the International Celestial Reference Frame, will be discussed: its
history, the use of the group delay as the measured quantity, the positional
accuracy of 0.3 milliarcsec, and possible future improvements. On the other
hand, for the determination of the motion of celestial objects, accuracies
approaching 0.01 milliarcsec can be obtained by measuring the differential
position between the target object and nearby stationary sources. This
astrometric technique uses phase referencing, and the current techniques and
limitations are discussed, using the results from four experiments. Brief
comments are included on the interpretation of the Jupiter gravity deflection
experiment of September 2002.Comment: 25 pages, 7 figures--Presented at JENAM meeting in Budapest, 27-30
August 200
Radio Tests of GR
Since VLBI techniques give microarcsecond position accuracy of celestial
objects, tests of GR using radio sources as probes of a gravitational field
have been made. We present the results from two recent tests using the VLBA: In
2005, the measurement of the classical solar deflection; and in 2002, the
measurement of the retarded gravitational deflection associated with Jupiter.
The deflection experiment measured PPN-gamma to an accuracy of 0.0003; the
Jupiter experiment measured the retarded term to 20% accuracy. The controversy
over the interpretation of the retarded term is summarized.Comment: 4 pages: IAU24
Sub-Milliarcsecond Precision of Pulsar Motions: Using In-Beam Calibrators with the VLBA
We present Very Long Baseline Array phase-referenced measurements of the
parallax and proper motion of two pulsars, B0919+06 and B1857-26.
Sub-milliarcsecond positional accuracy was obtained by simultaneously observing
a weak calibrator source within the 40' field of view of the VLBA at 1.5 GHz.
We discuss the merits of using weak close calibrator sources for VLBI
observations at low frequencies, and outline a method of observation and data
reduction for these type of measurements. For the pulsar B1919+06 we measure a
parallax of 0.31 +/- 0.14 mas. The accuracy of the proper motions is
approximately 0.5 mas, an order of magnitude improvement over most previous
determinations.Comment: 11 pages plus 4 figures. In press, Astronomical Journa
ALMA Temporal Phase Stability and the Effectiveness of Water Vapor Radiometer
Atacama Large Millimeter/submillimeter Array (ALMA) will be the world largest
mm/submm interferometer, and currently the Early Science is ongoing, together
with the commissioning and science verification (CSV). Here we present a study
of the temporal phase stability of the entire ALMA system from antennas to the
correlator. We verified the temporal phase stability of ALMA using data, taken
during the last two years of CSV activities. The data consist of integrations
on strong point sources (i.e., bright quasars) at various frequency bands, and
at various baseline lengths (up to 600 m). From the observations of strong
quasars for a long time (from a few tens of minutes, up to an hour), we derived
the 2-point Allan Standard Deviation after the atmospheric phase correction
using the 183 GHz Water Vapor Radiometer (WVR) installed in each 12 m antenna,
and confirmed that the phase stability of all the baselines reached the ALMA
specification. Since we applied the WVR phase correction to all the data
mentioned above, we also studied the effectiveness of the WVR phase correction
at various frequencies, baseline lengths, and weather conditions. The phase
stability often improves a factor of 2 - 3 after the correction, and sometimes
a factor of 7 improvement can be obtained. However, the corrected data still
displays an increasing phase fluctuation as a function of baseline length,
suggesting that the dry component (e.g., N2 and O2) in the atmosphere also
contributes the phase fluctuation in the data, although the imperfection of the
WVR phase correction cannot be ruled out at this moment.Comment: Proc. SPIE 8444-125, in press (7 pages, 4 figures, 1 table
A Catalog of the East-West Visibility Functions of Radio Sources at 1425 MHz
As part of a general program for determining the radio structure of discrete sources, 532 sources have been observed using the two 90-ft paraboloids at the Owens Valley Radio Observatory as an east-west interferometer. The frequency of the observations was 1425 MHz and up to nine east-west spacings, ranging from 144 wavelengths to 2626 wavelengths, were used.
Because of the large amount of data, the discussion of the observations and results will be given in several papers. This paper, the first in the series, will list the visibility functions of the data. The interpretation of a visibility function in terms of a source structure and the statistical interpretation of the structures will be given in following papers
A Catalog of the North-South Visibility Functions of Radio Sources at 1425 MHz
Observations with the North-South interferometer were made in 1966 using the two 90-foot paraboloids at the Owens Valley Radio Observatory to determine the strip-scan distribution in position angle 0° for a large number of extra-galactic radio sources. The frequency of the observations was 1425 MHz. In conjuction with other data taken with an East-West interferometer (Fomalong 1967; hereafter called Paper I) and off-transit data taken in 1966 and 1967, two-dimensional structures can be deduced for the sources. These results will be published elsewhere (small-diameter sources, Fomalont and Moffett 1971; large-diameter sources, Fomalont 1971)
The Measurement of the Light Deflection from Jupiter: Experimental Results
We have determined the relativistic light deflection of the quasar J0842+1835
as Jupiter passed within 3.7' on 2002 September 8, by measuring the time delay
using the VLBA and Effelsberg radio telescopes at 8.4 GHz. At closest approach,
General Relativity (GR) predicts a radial (static) deflection of 1190
microarcsec, and tangential (retarded) deflection in the direction of Jupiter's
motion of 51 microarcsec. Our experiment achieved an rms position error of <10
microarcsec, and measured this retarded deflection to be 0.98 +/- 0.19 (rms
error) times that predicted by GR. The increase positional accuracy for this
VLBI phase referencing experiment was achieved by using two calibrator sources.
Comments on the interpretation of this experiment are given
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