A Neutron Star Identification for the High-Energy Gamma-Ray Source 3EG J1835+5918 Detected in the [ITAL]ROSAT[/ITAL] All-Sky Survey

In the error box of 3EG J1835+5918, the brightest as-yet unidentified EGRETsource at intermediate Galactic latitude, we find a weak, ultrasoft X-raysource at energies E < 0.3 keV in the ROSAT All-Sky Survey. Deep opticalimaging at the location of this source, as pinpointed by an observation withthe ROSAT HRI, reveals a blank field to a limit of V > 25.2. The correspondinglower limit on f_X/f_V is 300, which signifies that the X-ray source isprobably a thermally emitting neutron star. Considering our previous completemultiwavelength survey of the 3EG J1835+5918 region which failed to find anyother notable candidate for identification with 3EG J1835+5918, we propose thatthis X-ray source, RX J1836.2+5925, is a rotation-powered gamma-ray pulsarwhich is either older or more distant than the prototype Geminga. We seemarginal evidence for variability between two ROSAT HRI observations. If real,this would indicate that the X-ray emission has an external origin, perhaps dueto intermittent heating of the polar caps by a variable particle accelerator.RX J1836.2+5925 could even be an old, recycled pulsar, which may neverthelesshave a high gamma-ray efficiency.Comment: 5 pages, accepted for publication in The Astrophysical Journal Letter