The long time-scale X-ray variability of the radio-quiet quasar PG 0804+761

We present the results from a study of the timing properties and the energy spectrum of the radio-quiet quasar PG 0804+761, based on monitoring RXTE/PCA observations that lasted for a year. This is a systematic study of the X-ray variations on time-scales of weeks/months of the most luminous radio-quiet quasar studied so far. We detect significant variations in the 2–10 keV band of an average amplitude of ∼15 per cent. The excess variance of the light curve is smaller than that of Seyfert galaxies, entirely consistent with the relationship between variability amplitude and luminosity defined from the Seyfert data alone. The power spectrum of the RXTE light curve follows a power-like form of slope ∼−1. However, when we extend the power spectrum estimation at higher frequencies using archival ASCA data, we find strong evidence for an intrinsic steepening to a slope of ∼−2 at around ∼1 × 10⁻⁶ Hz. This ‘break frequency’ corresponds to a time-scale of ∼10 d. The time-average energy spectrum is well fitted by a Γ∼ 2 power-law model. We also find evidence for an iron line at ∼6.4 keV (rest frame) with EW ∼ 110 eV, similar to what is observed in Seyfert galaxies. The flux variations are not associated with any spectral variation. This is the only major difference that we find when we compare the variability properties of PG 0804+761 with those of Seyfert galaxies. Our results support the hypothesis that the same X-ray emission and variability mechanism operates in both Seyfert galaxies and quasars.