2005A&A...432...15P

We present results of a systematic analysis of the XMM-Newton spectra of 40 quasars (QSOs) (z≤1.72) from the Palomar-Green (PG) Bright Quasar Survey sample (M_B←23). The sample includes 35 radio-quiet quasars (RQQs) and 5 radio-loud quasars (RLQs). The analysis of the spectra above 2keV reveals that the hard X-ray continuum emission can be modeled with a power law component with <Γ_2–12keV≥1.89±0.11 and <Γ_2–12keV≥1.63^+0.02_–0.01 for the RQQs and RLQs, respectively. Below 2keV, a strong, broad excess is present in most QSO spectra. This feature has been fitted with four different models assuming several physical scenarios. All tested models (blackbody, multicolor blackbody, bremsstrahlung and power law) satisfactorily fitted the majority of the spectra. However, none of them is able to provide an adequate parameterization for the soft excess emission in all QSOs, indicating the absence of a universal shape for this spectral feature. An additional cold absorption component was required only in three sources. On the other hand, as recently pointed out by Porquet et al. (2004A&A...422...85P) for a smaller sample of PG QSOs, warm absorber features are present in 50% of the QSO spectra in contrast with their rare occurrence (∼5-10%) in previous studies. The XMM-Newton view of optically selected bright QSOs therefore suggests that there are no significant differences in the X-ray spectral properties compared with those of the low-luminosity Seyfert 1 galaxies. Properties of the Fe Kα emission lines are presented in a companion paper.