2004A&A...417.1017Z

We report on deep UBVRI-imaging of the field of the 17.3 Myr radiopulsar PSR B0950+08 obtained with the ESO/VLT/FORS1. Firm detection of the candidate pulsar optical counterpart in the BVRI bands with the magnitudes B=27.06±0.35, V=27.05±0.15, R_c=26.49±0.10 and I_c=26.20±0.17 enabled us to study, for the first time, the broadband spectrum of the object. The derived ∼0.24" offset of the object from the radiopulsar position is insignificant within the 0.21" uncertainties of our astrometry and ∼0.75" seeing value during the observations. The positional coincidence and unusual colors of the candidate ensure us that we likely detect the optical emission from PSR B0950+08. The optical-near-UV spectrum of the pulsar has a negative slope and can be fitted by a power law F_ν∝ν^–α with α=0.65±0.40, suggesting nonthermal emission. Within errors the optical flux is consistent with the power law fit of the ROSAT spectrum from the X-ray counterpart. We analyze our results together with the available multiwavelength data on other isolated pulsars of different ages detected in the optical range and find a significant correlation between their optical and 2-10 keV X-ray luminosities. This implies an origin of the nonthermal emission in both spectral domains. These objects show a significantly non-monotonic evolution of the efficiency of the optical-X-ray photon production from the pulsar spindown power, with a pronounced minimum at the beginning of the middle-age epoch and comparably high efficiencies of younger and older pulsars. This suggests different sensitivities of the photon production processes to the Goldreich-Julian current in magnetospheres of different age pulsars.