Other
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........20c&link_type=abstract
Thesis (PH.D.)--HARVARD UNIVERSITY, 1995.Source: Dissertation Abstracts International, Volume: 56-12, Section: B, page: 6792.
Other
9
Scientific paper
Very Long Baseline Interferometry (VLBI) has long been used to image compact radio sources with sub-milliarcsecond resolution. Additionally, interleaved VLBI observations of two or more sources that lie near each other on the plane of the sky can determine their relative positions, also with sub-milliarcsecond standard errors. These two capabilities of VLBI provide a means to determine the distances to certain classes of astrophysically interesting objects. A series of interleaved VLBI observations of a pulsar and one or more extragalactic reference sources can yield the parallax, proper motion, and position of the pulsar, with an uncertainty in the model-independent parallax distance as small as 5%. These results, coupled with independent pulse dispersion measurements, can help investigate the distribution of free electrons in the interstellar medium, and can help calibrate the dispersion-based galactic distance scale in the solar neighborhood. We have detected nine pulsars in up to four sessions of observation each, and have focused analysis on two pulsars that have dispersion -based distances of 170 and 1200 parsecs. For rm PSR2021{+}51, our estimate of proper motion is mux= -8.12+/-0.22 mas rm yr^{-1}, mu_ {y}={+}13.40+/-0.23 mas rm yr^{-1}; of parallax, pi=0.93+/-0.37 mas; and of kinetic age, tauk=2.0 +/-1.2 Myr. For PSR 1929{+}10, our preliminary (i.e., group-delay based) estimate of proper motion is mux= {+}96.7+/-1.7 mas rm yr^{-1}, mu_ {y}={+}41.3+/-3.5 mas rm yr^{-1}; of parallax, pi=5.0+/-1.5 mas; and of kinetic age, tauk=1.1+/-1.2 Myr. On an extremely different distance scale, VLBI observations can monitor the structure of the images of a gravitational lens system as well as the inter-image angular separation. Thereby, they can contribute towards determining the time delay (Delta tau) between the images and towards placing constraints on the mass distribution within the lens. These elements are both required to obtain an estimate of the Hubble constant (H_0) from a gravitational lens. We have made VLBI observations of two images of the gravitationally lensed quasar 0957{+}561 that attack both elements: two sessions aimed at detection of correlated structural evolution within the images to use as a basis for estimating Delta tau, and two additional sessions, separated by a time corresponding to an empirically-determined Delta tau, to investigate spatial variations in the relative magnification field between the images to use as a basis for placing constraints on the lens mass distribution. We have detected no significant proper motion between the core and inner-jet components in either image, but our VLBI observations appear sensitive enough to detect image-plane spatial gradients in the lens relative magnification field. Variations in the flux density of the inner regions of the two images from our four sessions of observations provide a time-delay estimate of Delta tau ~1 yr.
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