Bignami, GiovanniGiovanniBignami2020-09-172020-09-171998-01-01https://hdl.handle.net/20.500.13025/476Accuracy 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). We report here on a high-accuracy measurement of the optical position of Geminga (V=25.5), the only known radio-quiet INS. The procedure combines the Hipparcos and Tycho catalogues, ground-based astrometric data,and Hubble Space Telescope (HST) Wide Field Planetary Camera (WFPC2) images, to yield Gemingas absolute position to within 40 mas (per coordinate). Such a positional accuracy, unprecedented for the optical position of a pulsar or 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 star revolutions. Although quite a difficult task, this is the only way to improve our knowledge of the timing parameters of this radio silent INS.AstrophysicsG. F. Bignami: ASI authorsArticle Journalhttp://arxiv.org/abs/astro-ph/971102954dcce078580fe1368eeaed8