The Next Geminga: Search for Radio and X-ray Pulsations from the Neutron Star Identified with 3EG J1835+5918

Abstract

We report unsuccessful searches for pulsations from the neutron star RX J1836.2+5925 identified with the EGRET source 3EG J1835+5918. A 24 hr observation with the NRAO Green Bank Telescope at 820 MHz placed an upper limit on flux density of 17 uJy for P > 10 ms, and gradually increasing limits for 1 < P < 10 ms. The equivalent luminosity is lower than that of any known pulsar with the possible exception of the radio-quiet gamma-ray pulsar Geminga. A set of observations with the Chandra X-ray Observatory HRC totaling 118 ks revealed no pulsar with 1 ms < P < 10 s. The upper limit on its pulsed fraction is 35% assuming a sinusoidal pulse shape. The position of RX J1836.2+5925 in Chandra observations separated by 3 years is unchanged within errors, leading to an upper limit on its proper motion of <0.14"/yr, or v < 530 km/s at d = 800 pc, a maximum distance estimated from its thermal X-ray spectrum. With these null results, the properties of 3EG J1835+5918 and its X-ray counterpart RX J1836.2+5925 are consistent with a more distant or older version of Geminga, or perhaps a recycled pulsar. Having nearly exhausted the capabilities of current instrumentation at all wavelengths, it will likely fall to the Gamma-ray Large Area Space Telescope to discover pulsations from 3EG J1835+5918.