  We describe Chandra, HST, and radio observations that reveal a radio-quiet
but magnetospherically active neutron star in the error circle of the
high-energy gamma-ray source 3EG J1835+5918, the brightest of the unidentified
EGRET sources at high Galactic latitude. A Chandra ACIS-S spectrum of the
ultrasoft X-ray source RX J1836.2+5925, suggested by Mirabal & Halpern as the
neutron star counterpart of 3EG J1835+5918, requires two components: a
blackbody of T~3x10^5 K and a hard tail that can be parameterized as a power
law of photon index Gamma~2. An upper limit of d < 800 pc can be derived from
the blackbody fit under an assumption of R = 10 km. Deep optical imaging with
the HST STIS CCD failed to detect this source to a limit of V > 28.5, thus
f_X/f_V > 6000 and d > 250 pc assuming the X-ray fitted temperature for the
full surface. Repeated observations with the 76 m Lovell telescope at Jodrell
Bank place an upper limit of < 0.1 mJy on the flux density at 1400 MHz for a
pulsar with P > 0.1 s, and < 0.25 mJy for a ~10 ms pulsar at the location of RX
J1836.2+5925. All of this evidence points to an older, possibly more distant
version of the highly efficient gamma-ray pulsar Geminga, as the origin of the
gamma-rays from 3EG J1835+5918.
