Geminga in the Space Telescope Era.

Abstract

The distance to Geminga inferred from the γ-ray flux, the interstellar absorption of the soft X-ray emission and the proper motion measured for its optical counterpart, range from tens of parsecs to an upper limit of severl hundreds of parsecs. These distance estimates, however, are based on a number of assumptions on the emission mechanisms, on the structure of the interstellar medium and on the intrinsic speed of this neutron star. The refurbished Hubble Space Telescope offers, for the first time, the possibility to assess directly the distance to Geminga through the measure of the annual parallactic displacement of the optical counterpart G''. The resulting value is 157^+59^_-34_pc. The repeated precise positioning of G'' yielded also, as a by product, a very accurate measure of its proper motion.