A multiwavelength search for a counterpart of the brightest unidentified gamma-ray source 3EG J2020+4017 (2CG 078+2)

In search of the counterpart to the brightest unidentified gamma-ray source, 3EG J2020+4017 (2CG 078+2), we report on new X-ray and radio observations of the gamma Cygni field with the Chandra X-Ray Observatory and with the Green Bank Telescope (GBT). We also report on reanalysis of archival ROSAT data. With Chandra it became possible for the first time to measure the position of the putative gamma-ray counterpart, RX J2020.2+4026, with subarcsecond accuracy and to deduce its X-ray spectral characteristics. These observations demonstrate that RX J2020.2+4026 is associated with a K field star and therefore is unlikely to be the counterpart of the bright gamma-ray source 2CG 078+2 in the SNR G78.2+2.1 as had been previously suggested. The Chandra observation detected 37 additional X-ray sources, which were correlated with catalogs of optical and infrared data. Subsequent GBT radio observations covered the complete 99% EGRET likelihood contour of 3EG J2020+4017 with a sensitivity limit of L-820 approximate to 0.1 mJy kpc(2), which is lower than most of the recent deep radio search limits. If there is a pulsar operating in 3EG J2020+4017, this sensitivity limit suggests that either the pulsar does not produce significant amounts of radio emission or that its geometry is such that the radio beam does not intersect with the line of sight. Finally, reanalysis of archival ROSAT data leads to a flux upper limit of f(X)(0.1-2.4 keV) < 1.8 x 10(13) ergs s(-1) cm(-2) for a putative pointlike X-ray source located within the 68% confidence contour of 3EG J2020+4017. Adopting the supernova remnant age of 5400 yr and assuming a spin-down to X-ray energy conversion factor of 10(-4), this upper limit constrains the parameters of a putative neutron star as a counterpart for 3EG J2020+4017 to be P greater than or similar to 160 (d 1.5 kpc)(-1) ms, P greater than or similar to 5 x 10(-13) (d 1.5 kpc(-1))(-1) s s(-1), and B-perpendicular to greater than or similar to 9 x 10(12) (d 1.5 kpc)(-1) G.