Subaru optical observations of the old pulsar PSR B 0950 + 08 ⋆

We report the B band optical observations of an old (τ ∼ 17.5 Myr) radiopulsar PSR B0950+08 obtained with the Suprime-Cam at the Subaru telescope. We detected a faint object, B=27. 07±0.16. Within our astrometrical accuracy it coincides with the radio position of the pulsar and with the object detected earlier by Pavlov et al. (1996) in UV with the HST/FOC/F130LP. The positional coincidence and spectral properties of the object suggest that it is the optical counterpart of PSR B0950+08. Its flux in the B band is two times higher than one would expect from the suggested earlier Rayleigh-Jeans interpretation of the only available HST observations in the adjacent F130LP band. Based on the B and F130LP photometry of the suggested counterpart and on the available X-ray data we argue in favour of nonthermal origin of the broad-band optical spectrum of PSR B0950+08, as it is observed for the optical emission of the younger, middle-aged pulsars PSR B0656+14 and Geminga. At the same time, the optical efficiency of PSR B0950+08, estimated from its spin-down power and the detected optical flux, is by several orders of magnitude higher than for these pulsars, and comparable with that for the much younger and more energetic Crab pulsar. We cannot exclude the presence of a compact, ∼ 1′′, faint pulsar nebula around PSR B0950+08, elongated perpendicular to the vector of its proper motion, unless it is not a projection of a faint extended object on the pulsar position.