Multifrequency studies of the enigmatic gamma-ray source 3EG: J1835+5918

The EGRET telescope aboard the NASA Compton Gamma-Ray Observatory (CGRO) ha s repeatedly detected 3EG J1835+5918, a bright and steady source of high-en ergy gamma-ray emission which has not yet been identified. The absence of a ny likely counterpart for a bright gamma-ray source located 25 degrees off the Galactic plane initiated several attempts of deep observations at other wavelengths. We report on counterparts in X-rays on a basis of a 60-ks ROS AT HRI image. In older to conclude on the plausibility of the X-ray counter parts, we reanalysed data from EGRET at energies above 100 MeV and above 1 GeV, including data up to CGRO observation cycle 7. The gamma-ray sauce loc ation represents the latest and probably the final positional assessment ba sed on EGRET data. We especially address the question of flux and spectral variability, here discussed using the largest and most homogeneous data set available at high-energy gamma-rays for many years. The results from X-ray and gamma-ray observations were used in a follow-up optical identification campaign at the 2.2-m Guillermo Hare Telescope at Cananea, Mexico. VRI ima ging. has been performed at the positions of all of the X-ray counterpart c andidates, and spectra were taken where applicable. The results of the mult ifrequency identification campaign toward this enigmatic unidentified gamma -ray source are given, especially on the one object which might be associat ed with the gamma-ray source 3EG J1835+5918. This object has the characteri stics of an isolated neutron star and possibly of a radio-quiet pulsar.