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G.F. Bignami, P. A. Caraveo, R. Mignani, J. Edelstein, S. Bowyer
A new set of ground-based optical, HST UV and EUVE data on Geminga are presented. The object, identified with a high proper motion isolated neutron star (INS), is seen to emit thermal radiation with a temperature between (2.2-2.8)E+5 K. This is compatible with the global thermal emission from the surface of a standard neutron star of Geminga's spindown age located at the distance of about 160 pc, recently measured through its annual parallax. However, in the "optical" (3400-8000 A) range, combined ground based and HST data suggest the presence of a spectral feature (either in emission or possibly in absorption) superimposed to the Rayleigh-Jeans continuum. Its simplest interpretation could be related to an ion (H or He) cyclotron frequency for the object's canonical magnetic field of 1.5E+12 G. This is the first observational evidence for an optical spectral feature in an INS. A search for a similar effect should be feasible also for PSR0656+14, the only other INS possibly showing optical thermal emission from its surface.
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