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Paper: A Deep Optical Observation of an Enigmatic Unidentified Gamma-Ray Source 3EG J1835+5918
Volume: 289, The Proceedings of the IAU 8th Asian-Pacific Regional Meeting, Volume I
Page: 311
Authors: Totani, T.; Kawasaki, W.; Kawai, N.
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 about 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 about 28, although their detections are not firm because of a low signal-to-noise ratio. If the extended object of B about 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_X/f_B > 2700, giving further strong support for the neutron-star identification of 3EG J1835+5918. Interestingly, if either of the two sources at B about 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 about 4 x 10^5 K and about 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 to that of Geminga. x
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