Synchrotron X-ray emission from old pulsars

We study the synchrotron radiation as the observed non-thermal emission by the X-ray satellites from old pulsars (greater than or similar to 1-10 Myr) to investigate the particle acceleration in their magnetospheres. We assume that the power-law component of the observed X-ray spectra is caused by the synchrotron radiation from electrons and positrons in the magnetosphere. We consider two pair-production mechanisms of X-ray emitting particles, the magnetic and the photon-photon pair productions. High-energy photons, which ignite the pair production, are emitted via the curvature radiation of the accelerated particles. We use the analytical description for the radiative transfer and estimate the luminosity of the synchrotron radiation. We find that for pulsars with the spin-down luminosity L-sd less than or similar to 10(33) erg s(-1), the locations of the particle acceleration and the non-thermal X-ray emission are within less than or similar to 10(7) cm from the centre of the neutron star, where the magnetic pair production occurs. For pulsars with the spin-down luminosity L-sd less than or similar to 10(31) erg s(-1) such as J0108-1431, the synchrotron radiation is difficult to explain the observed non-thermal component even if we consider the existence of the strong and small-scale surface magnetic field structures.