The X-ray emission properties of millisecond pulsars

Until now X-radiation from nine millisecond pulsars has been detected. In the present paper we summarize the observations and show the results of a re-analysis of archival ROSAT data. In addition we present the results of recent observations of PSR J0437-4715 with the ROSAT PSPC and HRI detectors. We show that the pulsed fraction is independent of energy in the range 0.1-2.4 keV. The pulse width as measured at X-ray energies is comparable with that observed in the radio domain. An upper limit for the X-ray luminosity of the pulsar's bow-shock visible in H_alpha is found to be L_x < 2 x 10^29 erg/s (d/180pc)^2. We further report on the discoveries of PSR J1024-0719, J1744-1134 and J2124-3358 with the ROSAT-HRI, making them the first solitary galactic millisecond pulsars detected at X-ray energies. The pulse profile of PSR J2124-3358 shows marginal evidence for a double peak structure. We finally discuss the observed emission properties of the detected millisecond pulsars and conclude that the measured power-law spectra and pulse profiles together with the close correlation between the pulsar's spin-down energy and the observed X-ray luminosity suggests a non-thermal origin for the bulk of the observed X-rays.