The average 2-10 keV X-ray spectrum of low/medium-redshift quasars observed with EXOSAT (Comastri et al. 1992; Lawson et al. 1992), Ginga (Williams et al. 1992; Lawson & Turner 1997) and ASCA (Reeves et al. 1997; George et al. 2000) is well represented by a power law with a slope similar to that of lower luminosity Seyfert 1 galaxies ( 1.9-2.0, Nandra & Pounds 1994) and significant dispersion around the mean value 0.2-0.3. The typical imprints of reprocessing gas (i.e. the reflection "hump" and the K emission line), which characterize the spectra of Seyfert 1 galaxies (Nandra & Pounds 1994; Nandra et al. 1997) are however not present in high luminosity objects.
Indeed, only a small number of quasars have a spectrum more complex than a simple power law (plus, sometimes, a soft excess at low energies). This fact has been tentatively ascribed to different physical conditions of the accreting gas in the innermost regions surrounding high-luminosity quasars with respect to Seyfert galaxies. Iwasawa & Taniguchi (1993) suggested the existence (based on Ginga data) of an X-ray "Baldwin" effect, whereby the equivalent width of the iron K line decreases with luminosity. Nandra et al. (1997, 1999) have recently confirmed and extended this result using ASCA data. Both the profile and strenght of the K line change with luminosity. In particular, for luminosities of erg s-1, ionized iron lines have been detected in a few radio-quiet quasars (RQQs) (Nandra et al. 1996; Yamashita et al. 1997; George et al. 2000). At higher luminosities there is no evidence for any emission line at all (Nandra 1999; Vignali et al. 1999). Unfortunately, high-luminosity quasars are usually found at relatively high-redshift and therefore are rather weak in X-rays, making the study of spectral features extremely difficult.
High luminosity quasars differs from Seyfert 1s also in the observed shape of the X-ray continuum. Recent studies have suggested that high-redshift RQQs (Vignali et al. 1999) have flatter X-ray spectral slopes than lower redshift objects (George et al. 2000). However, it is not clear whether the spectral flattening is a function of redshift, implying a flattening of the continuum towards high energies, or rather it depends on the luminosity, thus suggesting a different emission mechanism at high luminosities.
Luminous radio-quiet quasars in the local Universe are rare. In this regard, PDS 456 is an exception and could provide hints on the nature of some of the properties described above, which are of great relevance to the physics of AGNs. PDS 456 is a bright (B=14.7) nearby (z = 0.184, Torres et al. 1997) radio-quiet quasar ( = 22.7 mJy, Condon et al. 1998; radio-loudness = -0.7, Reeves et al. 2000) close to the Galactic plane (l = 10o.39, b=11o.16; 2 1021 cm-2, Dickey & Lockman 1990). The bolometric luminosity (L 1047 erg s-1; Reeves et al. 2000) makes PDS 456 more luminous than the nearby (z = 0.158) radio-loud quasar 3C 273, without being jet-dominated. The ASCA and RXTE quasi-simultaneous observations discussed by Reeves et al. (2000) reveal the presence of a deep edge-like feature at E 8 keV which is likely to be due to highly ionized iron atoms. The 2-10 keV flux doubled its intensity during a strong outburst which lasted for about 4 hours.
In order to further investigate the peculiar properties of this quasar we have carried out a medium-deep observation of PDS 456 with BeppoSAX. The results of the analysis are presented in Sect. 2, compared with a re-analysis of the ASCA observation in Sect. 3 and discussed in Sect. 4. Throughout the paper a Friedmann cosmology with H0 = 50 km s-1 Mpc-1 and q0 = 0.0 is assumed.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 19100