Bignami, GiovanniGiovanniBignamiMignani, R. P.R. P.MignaniCaraveo, P. A.P. A.Caraveo2020-09-172020-09-171998-02-01https://hdl.handle.net/20.500.13025/552New optical and near-UV HST observations of Geminga are presented. When compared with previous ground-based and HST imaging, the data confirm and better define the presence of an emission feature centered at 6,000 Angstrom and superimposed on the thermal continuum best fitting the extreme-UV/soft X-ray data. This feature may be interpreted in terms of cyclotron emission originated from a mixture of H/He ions in the neutron stars atmosphere. In the case of pure Hydrogen, the feature wavelength would imply a magnetic field of order 3-5 10d11 G, consistent with the value deduced from the dynamical parameters of the pulsar. If due to a cyclotron emission, the observation of this feature would represent the first case of an in situ measurement of the surface magnetic field of an isolated neutron star.G. F. Bignami: ASI authorsArticle Journalhttp://arxiv.org/abs/astro-ph/980215854dcce078580fe1368eeaefa