Multiwavelength data suggest a cyclotron feature on the hot thermal continuum of Geminga

A new set of ground-based optical, HST, UV, and EWE 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 in the range (2.2-2.8) x 10(5) K. This is compatible with the global thermal emission from the surface of a standard neutron star of Geminga's spin-down age located at the distance of about 160 pc, recently measured through its annual parallax. However, in the ''optical'' (3400-8000 Angstrom) range, combined ground-based and HST data suggest the presence of a spectral feature (either in emission or possibly in absorption) superimposed on 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 1.5 x 10(12) G. This is the first observational evidence for an optical spectral feature in an INS. A search for a similar effect should also be feasible for PSR 0656+14, the only other INS possibly showing optical thermal emission from its surface.