ON THE RELATIONSHIP BETWEEN THE OPTICAL AND X-RAY LUMINOSITIES OF QUASARS

We address the question of the relationship between X-ray and optical luminosities for both optically and X-ray selected quasar samples. We apply a generalized regression algorithm for the case of samples involving censored data, with errors on both the dependent and the independent variable. Contrary to some previous results, we find that such relationship is consistent with being a linear one (L(x) proportional to L(o)). We argue that previous reports of non-linear relationships (i.e. L(x) proportional to L(o)(e) with e < 1) are due to the neglect of the influence of the photometric errors, the precise knowledge of which strongly influences the reliability of the results. Further progresses in the determination of the L(o) - L(x) relationship can be achieved with ROSAT observations of the new generation of large bright quasar surveys.