PDS 456: an extreme accretion rate quasar?

We present quasi-simultaneous ASCA and RXTE observations of the most luminous known active galactic nucleus in the local (z < 0.3) Universe, the recently discovered quasar PDS 456. Multiwavelength observations have been conducted that show that PDS 456 has a bolometric luminosity of similar to 10(47) erg s(-1) peaking in the ultraviolet part of the spectrum. In the X-ray band the 2-10 keV (rest-frame) luminosity is 10(45) erg s(-1). The broad-band X-ray spectrum obtained with ASCA and RXTE contains considerable complexity. The most striking feature observed is a very deep, ionized iron K edge, observed at 8.7 keV in the quasar rest-frame. We find that these features are consistent with reprocessing from highly ionized matter, probably the inner accretion disc, PDS 456 appeared to show a strong (factor of similar to 2.1) outburst in just similar to 17 ks, although non-intrinsic sources cannot be completely ruled out. If confirmed, this would be an unusual event for such a high-luminosity source, with a light-crossing-time corresponding to similar to 2R(S). The implication would be that flaring occurs within the very central regions, or else that PDS 456 is a 'super-Eddington' or relativistically beamed system. Overall we conclude on the basis of the extreme blue/UV luminosity, the rapid X-ray variability and from the imprint of highly ionized material on the X-ray spectrum, that PDS 456 is a quasar with an unusually high accretion rate.