On the peculiar X-ray properties of the bright nearby radio-quiet quasar PDS456

BeppoSAX and ASCA observations of the nearby (z=0.184), high-luminosity, radio-quiet quasar PDS456 are presented. The X-ray spectrum is characterized by a prominent ionized edge at 8-9 keV (originally discovered by RXTE, Reeves et al. 2000) and by a soft excess below 1.5 keV. The lack of any significant iron K alpha emission line suggests for the edge an origin from line-of-sight material rather than from reflection from a highly ionized accretion disc. The hard X-ray continuum is indeed well modelled by transmission through a highly-ionized medium with a large column density (N_H warm = 4.5 x 10^24 cm^-2) plus an additional cold absorber with a lower column density (N_H cold = 2.7 x 10^22 cm^-2).