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N. Mirabal, J.P. Halpern (Columbia U.)
\def\source{3EG~J1835+5918} \def\xray{RX~J1836.2+5925} \def\ro{ROSAT\/} \def\asca{ASCA\/}
The EGRET source \source\ is the brightest and most accurately positioned of the as-yet unidentified high-energy \gamma-ray sources at intermediate Galactic latitude (\ell,b=89\circ,25\circ). In the error box of \source, we find a weak, ultrasoft X-ray source at energies E < 0.3~keV in the \ro\ All-Sky Survey. Deep optical imaging at the location of this source, as pinpointed by an observation with the \ro\ HRI, reveals a blank field to a limit of V > 25.2. The corresponding lower limit on fX/fV is 300, which signifies that the X-ray source \xray\ is probably a thermally emitting neutron star. Its X-ray flux is 10--40 times fainter than that of Geminga, the EGRET source which it most resembles. Recently, we completed a multiwavelength study of the region around \source, including X-ray, radio, and optical imaging surveys, as well as optical spectroscopic classification of all of the active objects in this area (Mirabal et al. 2000, ApJ, 541, 180). Identifications were made for all of the ROSAT and ASCA sources, except for \xray, to a flux limit of approximately 5 \times 10-14~erg~cm-2~s-1, which is 10-4 of the \gamma-ray flux. Since none of these other objects are in any way notable candidates for association with \source, we propose that \xray\ is the correct identification, and that it is a rotation-powered \gamma-ray pulsar which is either older or more distant than the prototype Geminga. We see marginal evidence for variability between two \ro\ HRI observations. If real, this would indicate that the X-ray emission has an external origin, perhaps due to intermittent heating of the polar caps by a variable particle accelerator. \xray\ could even be an old, recycled pulsar, which may nevertheless have a high \gamma-ray efficiency.