PARALLAX OBSERVATIONS WITH THE HUBBLE-SPACE-TELESCOPE YIELD THE DISTANCE TO GEMINGA

Using data from Hubble Space Telescope observations, we report the fir st optical measurement of the annual parallax of a neutron star. This is the counterpart of the X-ray/gamma-ray pulsar Geminga, for which st rong proper motion has already been reported. Significant displacement from such motion is detected using WFPC2 observations, taken at the d ates of the maximum predicted parallactic factor. The resulting distan ce value is 157 pc (+59, -34), just consistent with the lower end of t he wide range derived from X-ray data. This measurement solves the lon g-standing problem of Geminga's distance and allows for the first prec ise determination of a neutron star luminosity from electron volts to giga-electron volts. Moreover, the repeated precise positionings subst antially improve our knowledge of its proper motion.