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Astron. Astrophys. 334, 498-504 (1998)

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Does the optical-to-X-ray energy distribution of quasars depend on optical luminosity?

W. Yuan, J. Siebert and W. Brinkmann

Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, D-85740 Garching, Germany

Received 18 September 1997 / Accepted 15 January 1998


We report on a detailed analysis of the correlation between the optical-UV and X-ray luminosities of quasars by means of Monte Carlo simulations, using a realistic luminosity function. We find, for a quasar population with an intrinsically constant, mean X-ray loudness [FORMULA], that the simulated [FORMULA] - [FORMULA] relation can exhibit various `apparent' properties, including an increasing [FORMULA] with [FORMULA], similar to what has been found from observations. The determining factor for this behavior turns out to be the relative strength of the dispersions of the luminosities, i.e. their deviations from the mean spectral energy distribution at the optical and X-ray bands, such that a dispersion larger for the optical luminosity than for the X-ray luminosity tends to result in an apparent correlation. We suggest that the observed [FORMULA] - [FORMULA] correlation can be attributed, at least to some extent, to such an effect, and is thus not an underlying physical property. The consequences of taking into account the luminosity dispersions in an analysis of the observed luminosity correlations is briefly discussed. We note that similar considerations might also apply for the Baldwin effect.

Key words: quasars: general – X-rays: general – methods: statistical

Send offprint requests to: W. Yuan


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© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998