AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 50. High Angular Resolution Science with the NRAO Very Long Baseline Array
Display, Tuesday, June 1, 1999, 10:00am-7:00pm, Southwest Exhibit Hall

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[50.01] VLBI Neutron Star Astrometry: Techniques and Initial Results

S. Chatterjee (Cornell / NRAO), J.M. Cordes (Cornell)

Radio pulsars form a population with high velocities and velocity dispersions. Multi-epoch VLBI astrometry on these objects leads to a determination of their proper motions with unprecedented accuracy, and provides constraints on supernova core-collapse processes as well as reference frame ties.

We present initial results from an astrometry program for a number of pulsars using the NRAO Very Long Baseline Array. Pulsar gating is used to boost the signal to noise ratio, and the observations are phase-referenced to preserve absolute positional information. At 1.4 GHz, we achieve position accuracies of ~0.5 mas, though we are limited by the residual ionosphere between our targets and calibrator sources, and errors can be as larger for particular observations.

One proven way to minimize this problem is to use an in-beam calibrator. A proper motion for B0919+06 is presented using five observations (over four epochs) calibrated in this way: we obtain a proper motion \mu\alpha = 17.7 ±0.3 mas/yr, \mu\delta = 79.2 ±0.5 mas/yr, and a preliminary parallax \pi = 0.31 ±0.14 mas (Fomalont et al. 1999; AJ, in press; astro-ph/9903042). We expect these numbers to improve with the addition of another two observations (at a fifth epoch), which are being processed at present.

When obtainable, parallaxes give absolute distances to pulsars, allowing calibration of galactic electron density models and the dispersion measure -- distance scale: we expect that parallax distances of the order of ~1 kpc will be measurable with current techniques.

For most targets, an in-beam calibrator is not readily available. We are exploring other techniques to calibrate the ionosphere, such as the use of dual frequency GPS data and ionospheric models broadcast in real time: the results from some of these tests are presented as well.

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