Astron. Astrophys. 319, 413-429 (1997)

8. Conclusions

We have presented the broad band properties of the largest sample of X-ray detected radio-loud quasars published so far. The large number of objects allowed the study of the class specific properties of these objects with high statistical significance.

We found correlations between the luminosities and the spectral properties which are often related to some general class properties of the objects, like the radio spectral index. For example, flat spectrum quasars are X-ray louder than steep spectrum objects. Although most of these correlations could be confirmed with formally very small errors and high statistical significances it is not always certain whether these correlations are genuine, caused by flux limitations of the underlying databases, or induced by a physically different, hidden, and not yet understood parameter. Especially the spectral properties (flat/steep radio spectrum, CSS, GPS) and the correspondingly derived parameters (radio loudness, core dominance) seem to be interrelated with the redshift z which introduces detection biases through luminosity K-corrections.

Most of the correlations found can be reconciled in the framework of viewing dependent emission in all frequency bands, however, a quantitative analysis still remains to be done. The X-ray, the radio, and eventually even the optical emission seems to consist of two separate components, an isotropic one with steep spectra and a `beamed' component with a flat spectrum. Whether this beaming is only a viewing angle effect or whether it includes additional Doppler boosting remains unclear.

Some questions remain open, for example the high detection rate of high z quasars and the lack of optically very bright, X-ray luminous sources. Whereas the small number of CSS sources appears to be inconspicuous in all correlations, perhaps with slightly higher ratios of radio- to X-ray and radio- to optical luminosities, the GPS objects are predominantly found at higher redshifts with high radio and X-ray luminosities.

Even pure selection effects might be important as we are discussing catalogued, previously identified objects only, which were often found due to some unusual or outstanding observational properties.

Finally we want to mention that ROSAT's soft energy band can introduce biases as well, as the emission of the spectral region of the blue bump moves out of the PSPC's energy window with increasing redshift.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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