Multiband optical observations of the old PSR B0950+08^*

¹ Observatorio Astronomico Nacional SPM, Instituto de Astronomia, UNAM, Ensenada, BC, Mexico
² Ioffe Physical Technical Institute, Politekhnicheskaya 26, St. Petersburg, 194021, Russia e-mail: kopts@astro.ioffe.ru;shib@stella.ioffe.ru
³ Departamento de Fisica, Universidad de Concepcion, Casilla 160, Concepcion, Chile e-mail: rmennick@stars.cfm.udec.cl
⁴ Special Astrophysical Observatory Russian Academy of Science, Nizhnii Arkhyz 369167, Russia e-mail: vkom@sao.ru
⁵ Isaac Newton Institute of Chile, SAO Branch, Russia

Corresponding author: S. V. Zharikov, zhar@astrosen.unam.mx

Received: 25 August 2003
Accepted: 5 December 2003

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 ± 0.35, ± 0.15, ± 0.10 and ± 0.17 enabled us to study, for the first time, the broadband spectrum of the object. The derived ~ offset of the object from the radiopulsar position is insignificant within the uncertainties of our astrometry and ~ 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 with ± 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.