HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF THE MIDDLE-AGED PULSAR-0656+14

Multiwavelength spectral observations of middle-aged (tau similar to 1 0(4)-10(6) yr) isolated pulsars enable one in principle to separate th ermal radiation emitted from the neutron star surface and nonthermal r adiation from its magnetosphere. We have previously detected the middl e-aged radio and soft X-ray pulsar 0656+14 with the Hubble Space Teles cope Faint Object Camera (FOC) long-pass filter F130LP. In the present Letter we report new FOC observations of this pulsar with three broad band filters: F430W, F342W, and F195W. The optical-UV spectral flux ca n be fitted with a two-component spectral model that combines a power- law spectrum with a Rayleigh-Jeans spectrum. The nonthermal component with the power-law index alpha = 1.4(+0.6)(-0.7) dominates at lambda g reater than or similar to 3000 Angstrom. The thermal component is char acterized by the Rayleigh-Jeans parameter, G = T-6(R-10/d(500))(2), wh ere T = 10(6)T(6) K is the brightness temperature, R-infinity = 10R(10 ) km is the neutron star radius as seen by a distant observer, and d = 500d(500) pc is the distance. For a plausible extinction, E(B-V) = 0. 03, we obtained G = 3.6(-2.0)(+1.6). The observed shape of the optical -UV spectrum of PSR 0656+14 differs drastically from those of both you nger pulsars (Crab, 0540-69, Vela) and the older pulsar Geminga.