8th HEAD Meeting, 8-11 September, 2004
Session 17 Neutron Stars and X-ray Binaries
Poster, Thursday, September 9, 2004, 9:00am-10:00pm

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[17.24] VLBA Astrometry of X-ray Binaries.

V. Dhawan, M. P. Rupen (NRAO Socorro, NM), E. B. Fomalont (NRAO Charlottesville, VA.), A. J. Mioduszewski (NRAO Socorro, NM), J.-F. Lestrade (Observatoire de Paris)

Very Long Baseline Arrays of radio interferometers attain sub-milliarcsecond astrometric accuracy - they define the celestial and terrestrial reference frames. Proper motions and parallaxes of pulsars are now being determined by several groups. The distances and velocities constrain the birthplace, age, and evolution of the neutron star.

In contrast, astrometry of X-ray binaries is not as well advanced: they are radio quiet but for the spectacular microquasars. The latter, besides their superluminal ejecta, show flat-spectrum AU-size synchrotron radio jets in the power-law dominated X-ray state. This AU-scale emission traces the motion of the accreting object, thus relating to questions such as: Do black holes receive a birth kick as do young pulsars? Millisecond pulsars are believed to be old NS rejuvenated by accretion spin-up. Observationally, they have lower space velocities compared to new-born pulsars. What is the corresponding distribution of space velocities of BH? Are very massive black holes (>7M\odot) born without going through a supernova, and hence slow moving? Is the binary part of the disk, the bulge, or the halo population? Is it born in the galactic plane or in a globular cluster and ejected after a close dynamical encounter?

We present first results from our recently initiated astrometric campaign, and discuss a few specific cases in more detail - a mixed bag with some runaways, some slowpokes, and perhaps a halo black hole.

{footnotesize The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.}

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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.