Optical study of the Geminga candidate field

Abstract

We have observed the field of the x-ray source IE 0630 + 178, proposed as a probable counterpart of the γ-ray source 2CG 195+04 (Geminga). Deep CCD images of the field show two very faint optical sources within the x-ray error circle, as well as the 21 mag star (hereafter referred to as the G candidate) proposed earlier as the possible optical counterpart. The G candidate does not have a proper motion greater than 0.15 arcsec yr^(-1). The spectrum of this object shows no strong emission or absorption features. We conclude that the G candidate is most probably a distant G dwarf or a nearby, cool, white dwarf. We believe that one of the fainter objects within the x-ray circle is more likely to be the true counterpart of the x-ray source, and possibly of the γ-ray source as well. This result is consistent with the interpretation of Geminga as a nearby neutron star, if all the γ rays are generated by some nonthermal mechanism. Deep Hα images do not reveal presence of any emission-line nebulosity (e.g., a supernova remnant).