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

Abstract

The EGRET source 3EG J1835+5918 is the brightest and most accurately positioned of the as-yet unidentified high-energy gamma-ray sources at high Galactic latitude (l,b=89,25). We present a multiwavelength study of the region around it, including X-ray, radio, and optical imaging surveys, as well as optical spectroscopic classification of most of the active objects in this area. The identified X-ray sources in or near the EGRET error ellipse are radio-quiet QSOs, a galaxy cluster, and coronal emitting stars. The radio sources inside the error ellipse are all fainter than 4 mJy at 1.4 GHz. In addition there are no flat-spectrum radio sources in the vicinity. Since no blazar-like or pulsar-like candidate has been found as a result of these searches, 3EG J1835+5918 must be lacking one or more of the physically essential attributes of these known classes of gamma-ray emitters. If it is an AGN it lacks the beamed radio emission of blazars by at least a factor of 100 relative to identified EGRET blazars. If it is an isolated neutron star, it lacks the steady thermal X-rays from a cooling surface and the magnetospheric non-thermal X-ray emission that is characteristic of all EGRET pulsars. If a pulsar, 3EG J1835+5918 must be either older or more distant than Geminga, and probably an even more efficient or beamed gamma-ray engine.

arXiv comment31 pages, 6 figures, Accepted for publication in ApJ, figures with finding charts excluded (version with finding charts is available from the authors)

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• Is the EGRET source 3EG J1621+8203 the radio galaxy NGC 6251?
• A Possible X-ray and Radio Counterpart of the High-Energy Gamma-ray Source 3EG J2227+6122
• A Neutron-Star Identification for the High-Energy Gamma-ray Source 3EG J1835+5918 Detected in the ROSAT All-Sky Survey
• The Next Geminga: Deep Multiwavelength Observations of a Neutron Star Identified with 3EG J1835+5918
• Multifrequency studies of the enigmatic gamma-ray source 3EG J1835+5918
• Multiwavelength Examination of the COS-B Field 2CG 075+00 Yields a Blazar Identification for 3EG J2016+3657
• Fermi Large Area Telescope observations of PSR J1836+5925
• Search for a Point-Source Counterpart of the Unidentified Gamma-Ray Source TeV J2032+4130 in Cygnus
• Chandra X-ray Observations of the Two Brightest Unidentified High Galactic Latitude Fermi-LAT gamma-ray Sources
• An AGN Identification for 3EG J2006-2321
• Unraveling the Nature of Unidentified High Galactic Latitude Fermi/LAT Gamma-ray Sources with Suzaku
• Gamma-ray Emission Properties from Mature Pulsars in the Galaxy and in the Gould Belt
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• The Multiwavelength Approach to Unidentified Gamma-Ray Sources
• Multifrequency Strategies for the Identification of Gamma-Ray Sources
• Region of Excessive Flux of PeV Cosmic Rays in the Direction Toward Pulsars PSR J1840+5640 and LAT PSR J1836+5925
• A Deep Optical Observation for an Enigmatic Unidentified Gamma-Ray Source 3EG J1835+5918