SIMBAD references

X-ray observations of AGN with Einstein, EXOSAT and Ginga have shown that the spectra of quasars in the energy range from 2 to 10keV can be approximately described by a single power law model with a photon index of 1.7-2.0. They also suggested that a soft X-ray excess component (below ≃1keV) is a common feature in many quasars. In order to investigate the properties of the soft excess in radio-loud quasars we analysed the data of the pointed ROSAT/PSPC observations of the six radio-loud quasars PG 0007+106, PKS 0135-247, QSO 0537-286, QSO 0923+392, PG 1225+317, 3C 273. To these objects we added also the data of the radio-quiet quasar PG 0804+761. In four cases (including the radio-quiet object) we find that the spectra in the PSPC sensitivity band are considerably steeper than the spectra above 2 keV, a clear signature for a soft X-ray excess. In the other three objects the soft component could be hidden by the interstellar absorption or by the large redshift (z>2) and can not be excluded. We point out the limits of the detectability of a soft component with the ROSAT/PSPC detector due to these effects. The comparison of different spectral models for the soft component (power law, thermal bremsstrahlung, black body) suggest that the soft component must decline very rapidly towards the higher energies.