The Likely detection of pulsed high-energy gamma-ray emission from millisecond pulsar PSR J0218+4232

We report circumstantial evidence for the first detec- tion of pulsed high-energy -ray emission from a millisecond pulsar, PSR J0218+4232, using data collected with the Ener- getic Gamma Ray Experiment (EGRET) on board the Comp- ton 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 be- tween 300 MeV and 1 GeV both sources contribute to the-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 3:5 modulation significance and with a peak separation of 0:45 similar to the 0.1-10 keV pulse pro- file. A comparison in absolute phase with the 610 MHz radio profile shows alignment of the -ray pulses with two of three ra- dio pulses. The luminosity of the pulsed emission (0.1-1 GeV) amounts L =1 :64 10 34 (=1 sr) (d=5:7 kpc) 2 erg s 1 which is 7% of the pulsar's total spin-down luminosity. The similarity of the X-ray and -ray pulse profile shapes of PSR J0218+4232, and the apparent alignment of the -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 pul- sars (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 -ray emission from radio pulsars can explain the -ray spectrum and luminosity of PSR J0218+4232.