A possible optical counterpart to the X-ray pulsar 1E 2259+586.

An intensified CCD observation of the field of the 3.49-sec X-ray pulsar 1E 2259+586 is discussed. The fainter of two stars lying several arcsec away from the nominal X-ray position determined by the High Resolution Imager camera of the Einstein Observatory is identified as the optical counterpart of the pulsar on the basis of recently detected IR pulsations. If the pulsar has a binary companion, it must be a low mass, main sequence dwarf or a degenerate star. The implication of the binary hypothesis is that the neutron star was formed in a Type I supernova explosion which occurred in a pre-existing close binary configuration.