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

1. Introduction

PSR J0218+4232 was discovered by Navarro et al. (1995) as a highly luminous 2.3 ms radio-pulsar in a 2.0 day orbit. This milli-second (ms) pulsar has an extremely broad pulse-profile and about half of the radio emission is unpulsed; this is taken by Navarro et al. (1995) as an indication that the magnetic field is almost aligned with the rotation axis. From the dispersion measure a lower limit on the distance of 5.7 kpc is derived.

Van Kerkwijk (1997) reported the detection of an optical counterpart for this recycled ms-pulsar. The pulsar mass function as given by Navarro et al. (1995) suggests a companion mass of .

Recently, Verbunt et al. (1996) detected soft X-ray emission from this source in a 22 ksec ROSAT HRI observation. A timing analysis yielded a 6% probability that the observed phase distribution was due to random processes. They also found indications for pulsed emission in an archival ROSAT PSPC observation with the pulsar at an unfavourable off-axis angle of . The significance for pulsed emission ranged from to depending on the size of the event extraction radius. Verbunt et al. (1996) conservatively reported a value of since the usage of an extraction radius is in this case not an optimum selection criterion in view of the "horse-shoe" - shaped far off-axis PSPC point spread function. Therefore the detection of pulsed emission was considered tentative. The ROSAT HRI and PSPC lightcurves suggested a double-peaked lightcurve.

Pulsed X-ray emission has been firmly detected sofar from only 3 ms-pulsars, namely PSR J0437-4715 (Becker & Trümper 1993), PSR B1821-24 (Saito et al. 1997) and PSR J2124-3358 (Becker & Trümper 1997).

It was also noticed by Verbunt et al. (1996) that the EGRET high-energy -ray source 2EG J0220+4228 (Thompson et al. 1995), detected above 100 MeV at a significance is positionally consistent with the pulsar. A timing analysis using archival EGRET MeV data yielded indications for pulsed emission above 100 MeV at significance levels slightly below , making 2EG J0220+4228 a potential high-energy gamma-ray counterpart for PSR J0218+4232.

In this paper the results are presented from a 100 ks follow-up observation of PSR J0218+4232 with the ROSAT XRT/HRI combination aimed at clarifying its soft X-ray (0.1-2.4 keV) timing properties.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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