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Astron. Astrophys. 359, 615-626 (2000)

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

L. Kuiper 1, W. Hermsen 1, F. Verbunt 2, D.J. Thompson 3, I.H. Stairs 4, A.G. Lyne 4, M.S. Strickman 5 and G. Cusumano 6

1 SRON-Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
2 Astronomical Institute, Utrecht University, 3508 TA Utrecht, The Netherlands
3 Code 661, Laboratory for High Energy Astrophysics, NASA Goddard Space Flight Center, Greenbelt, MD 20771, United States of America
4 University of Manchester, Jodrell Bank, Macclesfield SK11 9DL, UK
5 Naval Research Laboratory, Washington DC, United States of America
6 Istituto di Fisica Cosmica ed Applicazioni all'Informatica CNR, Via U. La Malfa 153, 90146, Palermo, Italy

Received 20 March 2000 / Accepted 8 May 2000

Abstract

We report circumstantial evidence for the first detection of pulsed high-energy [FORMULA]-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 [FORMULA]-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 [FORMULA] modulation significance and with a peak separation of [FORMULA] similar to the 0.1-10 keV pulse profile. A comparison in absolute phase with the 610 MHz radio profile shows alignment of the [FORMULA]-ray pulses with two of three radio pulses. The luminosity of the pulsed emission (0.1-1 GeV) amounts [FORMULA] which is [FORMULA]% of the pulsar's total spin-down luminosity. The similarity of the X-ray and [FORMULA]-ray pulse profile shapes of PSR J0218+4232, and the apparent alignment of the [FORMULA]-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 [FORMULA]-ray emission from radio pulsars can explain the [FORMULA]-ray spectrum and luminosity of PSR J0218+4232.

Key words: stars: pulsars: individual: PSR J0218+4232 – stars: neutron – galaxies: BL Lacertae objects: individual: 3C 66A – gamma rays: observations – X-rays: stars

Send offprint requests to: L. Kuiper (L.M.Kuiper@sron.nl)

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© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000
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