The Next Geminga: Deep Multiwavelength Observations of a Neutron Star Identified with 3EG J1835+5918

We describe Chandra, Hubble Space Telescope (HST), and radio observations that reveal a radio-quiet but magnetospherically active neutron star in the error circle of the high-energy γ-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∞ ≈ 3 × 105 K and a hard tail that can be parameterized as a power law of photon index Γ ≈ 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 Space Telescope Imaging Spectrograph CCD failed to detect this source to a limit of V > 28.5, and thus fX/fV > 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 less than 0.1 mJy on the flux density at 1400 MHz for a pulsar with P > 0.1 s and less than 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 γ-ray pulsar Geminga as the origin of the γ-rays from 3EG J1835+5918.