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The empirical relation between the X-ray luminosity (in the 2–10 keV band) and the rate of spin-down energy loss of a sample of 39 pulsars is re-examined considering recent data from ASCA, RXTE, BeppoSAX, Chandra, and XMM-Newton and including statistical and systematic errors. The data show a significant scatter around an average correlation between Lx, (2-10) and . By fitting a dependence of Lx, (2-10) on the period P and period derivative of the type , we obtain and (i.e. ). This translates into the relation with a normalization . However, the reduced is large (=7.2) making the fit unacceptable on statistical ground. All the X-ray luminosities lie below a critical line Lx,crit: the corresponding efficiency of conversion of rotational energy into 2–10 keV X-rays is and varies, within the sample, between 0.1 and 80%. The large dispersion of Lx below Lx,crit indicates that other physical parameters uncorrelated with P and need to be included to account for the observed emission at X-ray energies. We indicate a few possibilities that all conspire to reduce Lx, (2-10).
Possenti et al. (Fri,) studied this question.