On X-ray emission of radio pulsars

An analysis of distributions of some parameters of radio pulsars emitting X-ray radiation was carried out. The majority of such pulsars have short spin periods with mean value < P > = 133 msec. The distribution of period derivatives reveals a bimodality. One of the groups contains millisecond (recycled) pulsars with < log dP/dt > = -19.69. Pulsars with long periods and < log dP/dt > = -13.61 belong to the second group. Magnetic fields at the surface of the neutron star are characterized by a bimodal distribution aswell. Themean values of < log B-s > are 8.48 and 12.41, respectively, for the mentioned groups. The distribution ofmagnetic fields near the light cylinder does not show noticeable bimodality. The median value of log B-lc = 4.43 is almost three orders of magnitude higher compared with that for radio pulsars without a registered X-ray emission (< log B-lc > = 1.75). The median value of the loss of rotational energy (< log dE/dt > = 35.24) is also three orders of magnitude higher than the corresponding value for X-ray quiet radio pulsars. There is the expected strong correlation (the correlation coefficient is 0.97) between X-ray luminosity and loss of rotational energy, which are believed to be the main sources of all processes in pulsar magnetospheres. Dependence of X-ray luminosity on the magnetic field at the light cylinder has been detected. It supports the hypothesis that generation of non-thermal X-ray emission takes place at the periphery of the magnetosphere and is caused by the synchrotron mechanism. We detected positive correlations between luminosities in X-ray and gamma-ray ranges. Our results show that a purposeful search for pulsars in the hard electromagnetic ranges can be carried out for radio pulsars with high values of dE/dt and B-lc.