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 < 1 keV) and the hard (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 similar or equal to 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 similar to1 day, common to both energy bands. A longer (similar tomonths) timescale is detected in the soft X-ray band only.
Guainazzi, M., Loiseau, N., Matt, G., Orr, A. (2004). Timescales trips to PG quasars: The Mkn 478 case. PROGRESS OF THEORETICAL PHYSICS SUPPLEMENT(155), 243-246.