Hipparcos Positioning of Geminga : How and Why

Details Hipparcos Positioning of Geminga : How and Why Hipparcos Positioning of Geminga : How and Why

1997-11-04

arxiv.org/abs/astro-ph/9711029v1

Astronomy and Astrophysics

Astrophysics

4 pages, Latex, 2 postscript figures, Accepted for publication in Astronomy and Astrophysics

Scientific paper

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 (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 Geminga's 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.

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