Multiband optical observations of the old PSR B0950+08

Abstract

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, $R_{\rm c}=26.49$ ± 0.10 and $I_{\rm c}=26.20$ ± 0.17 enabled us to study, for the first time, the broadband spectrum of the object. The derived ~$0\farcs24$ offset of the object from the radiopulsar position is insignificant within the $ 0\farcs21$ uncertainties of our astrometry and ~$0\farcs75$ 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 ${F_\nu \propto \nu^{-\alpha}}$ with ${\rm \alpha=0.65}$ ± 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.