2001ApJ...547L.137M

In the error box of 3EG J1835+5918, the brightest as-yet unidentified EGRET source at intermediate Galactic latitude, we find a weak, ultrasoft X-ray source at energies E<0.3 keV in the ROSAT All-Sky Survey. Deep optical imaging at the location of this source, as pinpointed by an observation with the ROSAT HRI, reveals a blank field to a limit of V>25.2. The corresponding lower limit on f_X/f_V is 300, which signifies that the X-ray source is probably a thermally emitting neutron star. Considering our previous complete multiwavelength survey of the 3EG J1835+5918 region, which failed to find any other notable candidate for identification with 3EG J1835+5918, we propose that this X-ray source, RX J1836.2+5925, is a rotation-powered γ-ray pulsar that is either older or more distant than the prototype Geminga. We see marginal evidence for variability between two ROSAT 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. RX J1836.2+5925 could even be an old, recycled pulsar, which may nevertheless have a high γ-ray efficiency.