An atmospheric model for the ion cyclotron line of Geminga

Recent optical/UV data of Geminga (Mignani et al. 1998) have strengthen the significance of the emission feature present over the thermal continuum be st fitting the EUV/soft X-ray data. Here we present a phenomenological mode l interpreting the data in terms of an ion cyclotron feature, arising from a thin outer layer of hot plasma covering Geminga's polar caps. The width o f the feature favours for Geminga the strongly oblique rotator configuratio n, as already deduced from the soft X-ray data (Halpern & Ruderman 1993). O ur interpre ration implies a magnetic field of 3-5 10(11) G, consistent wit h the value deduced from the dynamical parameters of the pulsar. This would represent the first case of an independent measurement of the surface magn etic field of an isolated neutron star. The expected time modulation of Geminga's feature is also discussed.