We carried out a deep subarcsecond BRI imaging of the two middle-aged pulsars
to establish their properties in the optical range. Both pulsars are detected
at >10 sigma level. Geminga is for the first time reliably detected in the I
band with a magnitude of 25.10+/-0.14. We also reanalyze archival ESO/NTT and
HST broadband data and find that some published fluxes for Geminga were
estimated inaccurately. The resulting dereddened broadband spectra of both
pulsars are remarkably similar to each other and show significant flux
increases towards the far-UV and near-IR, and a wide flux excess in V-I bands.
This suggests a multicomponent structure of the optical emission. The
nonthermal power law component of the pulsar magnetospheric origin dominates in
the most part of the optical range. For PSR B0656+14 it is compatible with a
low energy extension of the power law tail seen in hard X-rays. For Geminga the
respective extension overshoots by a factor of 100 the nonthermal optical flux,
which has a less steep spectral slope than in X-rays. This implies a spectral
break at a photon energy of about 1 keV. The flux increases towards the far-UV
are compatible with contributions of the Rayleigh-Jeans parts of the blackbody
components from whole surfaces of the neutron stars dominating in soft X-rays.
The V-I excess, which is most significant for PSR B0656+14, suggests a third
spectral component of still unidentified origin. Faint, a few arcseconds in
size nebulae extended perpendicular to the proper motion directions of the
pulsars, are seen around both objects in our deepest I band images. They can be
optical counterparts of the bow-shock head of Geminga and of the tentative
pulsar wind nebula of PSR B0656+14 observed in X-rays.Comment: 15 pages, 8 figures, A&A, accepted. For higher resolution images see
http://www.ioffe.ru/astro/NSG/obs/map