A Deep Optical Observation for an Enigmatic Unidentified Gamma-Ray Source 3EG J1835+5918

Abstract

Abstract We report on a deep optical imaging observation using 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 $B\sim 26$, which 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, possible point sources at $B \sim 28$, although their detections are not firm because of a low signal-to-noise ratio. If the extended object of $B\sim 26$ is a galaxy and not related to 3EG J1835$+$5918, a lower limit on the X-ray/optical flux ratio can be set as $f_\mathrm{X}/f_B \gtrsim 2700$, giving further strong support for 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 $\sim 4 \times 10^5 \,\mathrm{K}$ and $\sim 300 \,\mathrm{pc}$, 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 $\mbox{3EG J1835$+$5918} / \mbox{RX J1836.2$+$5925}$ 3EG J1835$+$5918$/$RX J1836.2$+$5925 is at a similar level to that of Geminga.