[ITAL]Hubble Space Telescope[/ITAL] Observations of the Middle-aged Pulsar 0656+14

Multiwavelength spectral observations of middle-aged (τ~10 4 -10 6 yr) isolated pulsars enable one in principle to separate thermal radiation emitted from the neutron star surface and nonthermal radiation from its magnetosphere. We have previously detected the middle-aged radio and soft X-ray pulsar 0656+14 with the Hubble Space Telescope Faint Object Camera (FOC) long-pass filter F130LP. In the present Letter we report new FOC observations of this pulsar with three broadband filters: F430W, F342W, and F195W. The optical-UV spectral flux can 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 α=1.4 −0.7 + 0.6 dominates at λ≳3000 Å. The thermal component is characterized by the Rayleigh-Jeans parameter, G ≡ T 6 ( R 10 / d 500 ) 2 , where T =10 6 T 6 K is the brightness temperature, R =10 R 10 km is the neutron star radius as seen by a distant observer, and d =500 d 500 pc is the distance. For a plausible extinction, E ( B - V )=0.03, we obtained G =3.6 + 1.6 −2.0 . The observed shape of the optical-UV spectrum of PSR 0656+14 differs drastically from those of both younger pulsars (Crab, 0540-69, Vela) and the older pulsar Geminga.