Astronomy & Astrophysics

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Issue		A&A Volume 417, Number 3, April III 2004
Page(s)		1017 - 1030
Section		Stellar structure and evolution
DOI		10.1051/0004-6361:20034230

A&A 417, 1017-1030 (2004)
DOI: 10.1051/0004-6361:20034230

Multiband optical observations of the old PSR B0950+08

S. V. Zharikov¹, Yu. A. Shibanov², R. E. Mennickent³, V. N. Komarova^{4, 5}, A. B. Koptsevich² and G. H. Tovmassian¹

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

(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 B=27.06 $\pm$ 0.35, V=27.05 $\pm$ 0.15, $R_{\rm c}=26.49$ $\pm$ 0.10 and $I_{\rm c}=26.20$ $\pm$ 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 ${\rm\alpha=0.65}$ $\pm$ 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.

Key words: stars: pulsars: general -- stars: pulsars: individual: PSR B0950+08 -- stars: neutron

Offprint request: S. V. Zharikov, zhar@astrosen.unam.mx

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