The Importance of Positioning Isolated Neutron Stars: the Case of Geminga

We report on a multi-step procedure aimed at measuring, with high accuracy, the coordinates of Geminga within the Hipparcos and Tycho systems. Starting with an astrometric plate containing few tens of Tycho stars and 4 Hipparcos ones, we went through four intermediate steps to measure, to about 0.040 arcsec (per coordinate), the absolute position of Geminga in an HST Planetary Camera image. This positional accuracy, unprecedented for the optical position of an object this faint, is slightly better than that of the Crab pulsar, a comparable source of high energy gamma-rays but about 9 magnitudes brighter in the visible. This result, made possible by the availability of both Hipparcos and HST data, will allow significant refinement on the timing parameters of this source of copious gamma-ray emission.