Subaru optical observations of the two middle-aged pulsars PSR B0656+14 and Geminga

¹ Ioffe Physical Technical Institute, Politekhnicheskaya 26, St. Petersburg 194021, Russia e-mail: shib@astro.ioffe.ru
² Observatorio Astronomico Nacional SPM, Instituto de Astronomia, UNAM, Ensenada, BC, Mexico
³ Special Astrophysical Observatory of RAS, Karachai-Cherkessia, Nizhnij Arkhyz 369167, Russia
⁴ Isaac Newton Institute of Chile, SAO Branch, Russia
⁵ Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Muguro-ku, Tokyo 152-8551, Japan
⁶ RIKEN (Institute of Physical and Chemical Research) Hirosawa, Wako, Saitama 351-0198, Japan
⁷ University of Toronto, 60 St George St, Toronto, ON M5S1A7, Canada
⁸ Yamagata University, Yamagata 990-8560, Japan
⁹ Rikkyo University, Tokyo 171-8501, Japan

Received: 8 September 2005
Accepted: 17 October 2005

Abstract

Aims.We carried out a deep subarcsecond BRI imaging of the two middle-aged pulsars to establish their properties in the optical range. Methods.Astrometry and photometry methods are applied to identify the pulsars and to measure their fluxes. We also reanalyze archival ESO/NTT and HST broadband data and find that some published fluxes for Geminga were estimated inaccurately. The resulting dereddened broadband spectra in the near-IR-UV range are analyzed and compared with available data from the radio through gamma-rays.Results.Both pulsars are detected at 10σ level. Geminga is for the first time reliably detected in the I band with a magnitude of 25. The dereddened spectra of both pulsars are remarkably similar to each other and show significant flux increases towards the far-UV and near-IR, and a wide flux excess in  bands. This suggests a multicomponent structure of the optical emission. The nonthermal power law component of the pulsar magnetospheric origin dominates in the most part of the optical range. For PSR B0656+14  it is compatible with a low energy extension of the power law tail seen in hard X-rays. For Geminga  the respective extension overshoots by a factor of 100 the nonthermal optical flux, which has a less steep spectral slope than in X-rays. This implies a spectral break at a photon energy ~1 keV. The flux increases towards the far-UV are compatible with contributions of the Rayleigh-Jeans parts of the blackbody components from whole surfaces of the neutron stars dominating in soft X-rays. The  excess, which is most significant for PSR B0656+14, suggests a third spectral component of still unidentified origin. Faint, a few arcseconds in size nebulae extended perpendicular to the proper motion directions of the pulsars, are seen around both objects in our deepest I band images. They can be optical counterparts of the bow-shock head of Geminga and of the tentative pulsar wind nebula of PSR B0656+14  observed in X-rays.