Likely Detection of Pulsed High-energy Γ-rays from Millisecond Pulsar Psr J0218+4232

We report on the likely detection of pulsed high-energy γ-rays from the binary millisecond pulsar PSR J0218+4232 in 100-1000 MeV data from CGRO EGRET. Imaging analysis demonstrates that the highly significant γ-ray source 2EG J0220+4228 (∼ 10σ) is for energies > 100 MeV positionally consistent with both PSR J0218+4232 and the BL Lac 3C66A. However, above 1 GeV 3C66A is the evident counterpart , whereas between 100 and 300 MeV PSR J0218+4232 is the most likely one. Timing analysis using one ephemeris valid for all EGRET observations yields in the 100-1000 MeV range a double-pulse profile at a ∼ 3.5σ significance level. The phase separation is similar to the component separation of ∼ 0.47 observed at X-rays. A comparison of the γ-ray profile with the 610 MHz radio profile in absolute phase shows that the two γ-ray pulses coincide with two of the three emission features in the complex radio profile.