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Astron. Astrophys. 329, L1-L4 (1998)

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Letter to the Editor

Hipparcos positioning of Geminga: how and why * **

P.A. Caraveo 1, M.G. Lattanzi 2, G. Massone 2, R.P. Mignani 3, V.V. Makarov 4, M.A.C. Perryman 5 and G.F. Bignami 6, 1

1 Istituto di Fisica Cosmica del CNR, Milano, Italy
2 Osservatorio Astronomico di Torino, I-10025 Pino Torinese, Italy
3 Max-Plack-Institute für Extraterrestrische Physik, Garching, Germany
4 Copenhagen University Observatory, DK-1350 Copenhagen K, Denmark
5 Astrophysics Division, ESTEC, 2200AG Noordwijk, The Netherlands
6 Agenzia Spaziale Italiana, Via di Villa Patrizi 13, Roma, Italy

Received 10 July 1997 / Accepted 2 October 1997


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). We report here on a high-accuracy measurement of the optical position of Geminga ( [FORMULA] ), 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 Geminga's absolute position to within [FORMULA] (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 [FORMULA] -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.

Key words: space astrometry; pulsar; Geminga

* Based on Observation with the ESA Hipparcos satellite.
** Based on observations with the NASA/ESA Hubble Space Telescope.

Send offprint requests to: P. Caraveo, e-mail: pat@ifctr.mi.cnr.it

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© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997