We report a deep optical imaging observation by the Subaru telescope for a very soft X-ray source RX J1836.2+5925 , which has been suspected to be an isolated neutron star associated with the brightest as-yet unidentified EGRET source outside the Galactic plane , 3EG J1835+5918 .
An extended source having a complex , bipolar shape is found at B \sim 26 , and this might be an extended pulsar nebular whose flux is about 5-6 orders of magnitude lower than gamma-ray flux , although finding a galaxy of this magnitude by chance in the error circle is of order unity .
We have found two even fainter , possibly point sources at B \sim 28 , although their detections are not firm because of low signal-to-noise .
If the extended object of B \sim 26 is a galaxy and not related to 3EG J1835+5918 , a lower limit on X-ray/optical flux ratio is set as f _ { X } / f _ { B } \gtrsim 2700 , giving a further strong support of the neutron-star identification of 3EG J1835+5918 .
Interestingly , if either of the two sources at B \sim 28 is the real counterpart of RX J1836.2+5925 and thermal emission from the surface of an isolated neutron star , the temperature and distance to the source become \sim 4 \times 10 ^ { 5 } K and \sim 300 pc , respectively , showing a striking similarity of its spectral energy distribution to the proto-type radio-quiet gamma-ray pulsar Geminga .
No detection of nonthermal hard X-ray emission is consistent with the ASCA upper limit , if the nonthermal flux of 3EG J1835+5918/RX J1836.2+5925 is at a similar level with that of Gemiga .