  In search of the counterpart to the brightest unidentified gamma-ray source
3EG J2020+4017 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. 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 sub-arcsec 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 2CG078+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 ~ 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 the
pulsar either 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 < 1.8 x 10^-13 erg s^-1 cm^-2 for a putative point-like X-ray source
located within the 68% confidence contour of 3EG J2020+4017. Adopting the SNR
age of 5400 yrs 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 >= 160 (d/1.5 kpc)^-1 ms, P_dot >= 5 x
10^-13 (d/1.5 kpc)^-1 s s^-1 and B_perp >= 9 x 10^12 (d/1.5 kpc)^-1 G.
