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

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 ∼ 4.8σ level, showing a sharp main pulse with a duty cycle of ∼ 0.15 (350μs), compatible with smearing due to the scatter in the ROSAT clock calibration. The intrinsic width can be smaller than 100μs. There is an indication for a second peak at a phase separation of 0.47±0.01. The pulsed fraction is 37 ± 13%. 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 ∼ 14′′.