A search for the X-ray counterpart of the unidentified γ-ray source 3EG J2020+4017 (2CG078+2)

We report observations with the Chandra X-ray Observatory of a field in the γ-Cygni supernova remnant (SNR78.2+2.1) centered on the cataloged location of the unidentified, bright γ-ray source 3EG J2020+4017. In this search for an X-ray counterpart to the γ-ray source, we detected 30 X-ray sources. Of these, we found 17 strong-candidate counterparts in optical (visible through near-infrared) cataloged and an additional 3 through our optical observations. Based upon colors and (for several objects) optical spectra, nearly all the optically identified objects appear to be reddened main-sequence stars: None of the X-ray sources with an optical counterpart is a plausible X-ray counterpart to 3EG J2020+4017—if that γ-ray source is a spin-powered pulsar. Many of the 10 X-ray sources lacking optical counterparts are likely (extragalactic) active galactic nuclei, based upon the sky density of such sources. Although one of the 10 optically unidentified X-ray sources could be the γ-ray source, there is no auxiliary evidence supporting such an identification. Subject headings: gamma rays: individual (3EG J2020+4017) — ISM: individual (SNR78.2+2.1 = γ-Cygni SNR) — X rays: individual (various) NASA Marshall Space Flight Center, Space Science Office, VP62, Huntsville, AL 35812 USA. Universities Space Research Association, NASA Marshall Space Flight Center, Space Science Office, VP62, Huntsville, AL 35812 USA. Instituto Nacional de Astrof́ısica, Óptica y Electrónica, Luis Enrique Erro 1, Tonantzintla, Puebla 72840, México. Visiting Astronomer, Observatorio Astrof́ısico Guillermo Haro, Cananea, Sonora, México. Pappalardo Fellow; Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 37-664H, 77 Massachusetts Avenue, Cambridge, MA 02139 USA. Max-Planck-Institut für extraterrestrische Physik, 85741 Garching bei München, Germany.