Search for the Identification of 3EG J1835+5918: Evidence for a New Type of High‐Energy Gamma‐Ray Source

The EGRET source 3EG J1835+5918 is the brightest and most accuratelypositioned of the as-yet unidentified high-energy gamma-ray sources at highGalactic latitude (l,b=89,25). We present a multiwavelength study of the regionaround it, including X-ray, radio, and optical imaging surveys, as well asoptical spectroscopic classification of most of the active objects in thisarea. The identified X-ray sources in or near the EGRET error ellipse areradio-quiet QSOs, a galaxy cluster, and coronal emitting stars. The radiosources inside the error ellipse are all fainter than 4 mJy at 1.4 GHz. Inaddition there are no flat-spectrum radio sources in the vicinity. Since noblazar-like or pulsar-like candidate has been found as a result of thesesearches, 3EG J1835+5918 must be lacking one or more of the physicallyessential attributes of these known classes of gamma-ray emitters. If it is anAGN it lacks the beamed radio emission of blazars by at least a factor of 100relative to identified EGRET blazars. If it is an isolated neutron star, itlacks the steady thermal X-rays from a cooling surface and the magnetosphericnon-thermal X-ray emission that is characteristic of all EGRET pulsars. If apulsar, 3EG J1835+5918 must be either older or more distant than Geminga, andprobably an even more efficient or beamed gamma-ray engine.Comment: 31 pages, 6 figures, Accepted for publication in ApJ, figures with finding charts excluded (version with finding charts is available from the authors