We report circumstantial evidence for the first detection of pulsed high-energy \gamma -ray emission from a millisecond pulsar , PSR J0218 + 4232 , using data collected with the Energetic Gamma Ray Experiment ( EGRET ) on board the Compton Gamma Ray Observatory ( CGRO ) .
The EGRET source 3EG J0222+4253 is shown to be spatially consistent with PSR J0218 + 4232 for the energy range 100 - 300 MeV .
Above 1 GeV the nearby BL Lac 3C 66A is the evident counterpart , and between 300 MeV and 1 GeV both sources contribute to the \gamma -ray excess .
Folding the 100-1000 MeV photons with an accurate radio ephemeris of PSR J0218 + 4232 yields a double peaked pulse profile with a \sim 3.5 \sigma modulation significance and with a peak separation of \sim 0.45 similar to the 0.1-10 keV pulse profile .
A comparison in absolute phase with the 610 MHz radio profile shows alignment of the \gamma -ray pulses with two of three radio pulses .
The luminosity of the pulsed emission ( 0.1-1 GeV ) amounts L _ { \gamma } = 1.64 \cdot 10 ^ { 34 } \cdot ( \Delta \Omega / 1 \hbox { sr } ) \cdot ( d / 5.7 % \hbox { kpc } ) ^ { 2 } \hbox { erg s } ^ { -1 } which is \sim 7 % of the pulsar ’ s total spin-down luminosity .
The similarity of the X-ray and \gamma -ray pulse profile shapes of PSR J0218 + 4232 , and the apparent alignment of the \gamma -ray pulses with two radio pulses at 610 MHz , bears resemblance to the well-known picture for the Crab pulsar .
This similarity , and the fact that PSR J0218 + 4232 is one of three millisecond pulsars ( the others are PSR B1821-24 and PSR B1937+21 ) which exhibit very hard , highly non-thermal , high-luminosity X-ray emission in narrow pulses led us to discuss these millisecond pulsars as a class , noting that each of these has a magnetic field strength near the light cylinder comparable to that for the Crab .
None of the current models for \gamma -ray emission from radio pulsars can explain the \gamma -ray spectrum and luminosity of PSR J0218 + 4232 .