Astron. Astrophys. 336, 545-552 (1998)

Detection of pulsed X-ray emission from the binary millisecond pulsar PSR J0218+4232

L. Kuiper ¹, W. Hermsen ¹, F. Verbunt ² and T. Belloni <sup>3

1</sup> SRON-Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
² Astronomical Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
³ Astronomical Institute Anton Pannekoek and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received 23 January 1998 / Accepted 18 April 1998

Abstract

We report the detection of pulsed X-ray emission from the binary millisecond pulsar PSR J0218+4232 in a 100 ks ROSAT HRI observation. The lightcurve deviates from a flat distribution at a level, showing a sharp main pulse with a duty cycle of , compatible with smearing due to the scatter in the ROSAT clock calibration. The intrinsic width can be smaller than . There is an indication for a second peak at a phase separation of . The pulsed fraction is . No indication for a modulation at the orbital period of the binary system is found. This is the fourth millisecond pulsar showing modulation in X-rays at the radio-pulsar period. The sharp pulses, very similar to those of PSR B1821-24 and the Crab pulsar, point to a magnetospheric origin of the pulsed X-ray emission. The observed and derived parameters of PSR J0218+4232 and PSR B1821-24 are very consistent with the exception of the small pulsed fraction of PSR J0218+4232. The large unpulsed component in X-rays and that reported earlier in the radio domain can be explained by emission from a compact nebula with diameter .

Key words: pulsars: individual: PSR J0218+4232; PSR B1821-24 stars: neutron X-rays: stars

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998

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