Timescales Trips to PG Quasars: The Mkn 478 Case

The X-ray bright PG quasar PG 1440+356 (Mkn 478) was observed four times across 13 months with the XMM-Newton observatory. Flux variations with a dynamical range of a factor about 6 were observed in both the soft (E</it> ≪ 1 keV) and the hard (E</it> > 3 keV) X-ray band. However, large spectral variations are observed in the latter band only. We suggest that the X-ray emission in this object is due to Comptonization of k</it>T</it> ≃60 eV disk photons by a 20–50 keV plasma with a complex density structure. Analysis of the harmonic content of Mkn 478 light curves unveils a characteristic timescale ∼1 day, common to both energy bands. A longer (∼months) timescale is detected in the soft X-ray band only.