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

We report circumstantial evidence for the first detection of pulsed high-energy γ-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 γ-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 ∼ 3.5σ modulation significance and with a peak separation of ∼ 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 γ-ray pulses with two of three radio pulses. The luminosity of the pulsed emission (0.1–1 GeV) amounts Lγ = 1.64 · 1034 · (∆Ω/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 γ-ray pulse profile shapes of PSR J0218+4232, and the apparent alignment of the γ-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 γ-ray emission from radio pulsars can explain the γ-ray spectrum and luminosity of PSR J0218+4232.