ROSAT observations of the binary millisecond pulsar PSR J0751+1807

We report the discovery of the binary millisecond pulsar PSR J0751+1807 in the soft X-ray domain with ROSAT. The low number of source counts does not allow a detailed spectral modelling. Assuming an energy spectrum similar to that of PSR J0437—4715 implies, for the pulsar X-ray flux, fx ∼ 1 × 10⁻¹³ erg s⁻¹cm⁻² within the 0.1–2.4 keV energy range. This corresponds to an X-ray luminosity of Lx ∼ 4.7 × 10³¹ erg s⁻¹ if we adopt the dispersion measure based pulsar distance of 2 kpc. A photon arrival time analysis does not reveal significant X-ray pulses. A sine-wave fit to the folded light curve results in a 2σ pulsed fraction upper limit of 35 per cent.