On the accurate positioning of Geminga

Accuracy in the absolute position in the sky is one of the limiting factors for pulsar timing, and timing parameters have a direct impact on the understanding of the physics of Isolated Neutron Stars (INS). This is particularly true for Geminga, the only example of radio silent neutron star known so far. We have combined the Hipparcos and Tycho catalogues, ground-based astrometric data,and Hubble Space Telescope (HST) Wide Field Planetary Camera (WFPC2) images, to yield for the m(v) = 25.5 optical counterpart of Geminga a similar to 40 mab (per coordinate) uncertainty. Such a positional accuracy, unprecedented for the optical position of an object this faint, is needed to combine in phase gamma-ray photons collected over more than 20 years, i.e. over 2.5 billions of the star's revolutions.