Abstract
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 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 kT ≃ 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.
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