6.1. Quasars

There is no evidence of an iron line or any reflection features in the spectra of most luminous quasars (Nandra et al 1995; 1997). The equivalent width of the iron line is observed to decrease with increasing luminosity as one moves from the Seyfert 1 regime (L_X 10⁴⁴ erg s^-1) to the quasar regime, a phenomenon termed the `X-ray Baldwin effect' (Iwasawa & Taniguchi 1993; Nandra et al 1995). It has been suggested that the accretion disk becomes increasingly ionized. This might be due to the most luminous objects possessing accretion rates closer to the Eddington limit. One puzzling aspect of this explanation is that the disk must jump from being `cold' to being completely ionized otherwise we would observe instances of intermediate ionization in which the iron in the surface layers of the disk is H or He-like and the equivalent width of the line is even larger (Fig. 2 with = 1 x 10³). There should also be a large absorption edge that is not seen. As mentioned in Section 2.1, a thermal instability in the surface layers of the disk (Nayakshin et al 2000) may lead to the formation of a highly ionized blanket and circumvent this problem.

The absence of reflection features can also be explained if the fluorescing accretion disk subtends less than 2 sr as seen from the illuminating X-ray source, e.g. if the fluorescing accretion disk truncates at a few tens of Schwarzschild radii. A transition to an advection dominated accretion flow (ADAF), which is hot and optically-thin, would produce such a geometry. However, such structures can only exist at small acretion rates. Together with the fact that most of the energy in such a structure is advected into the black hole, ADAFs are expected to be much less luminous than most quasars. A more likely possibility is that, as an object approaches its Eddington limit, the region of the disk that is radiation pressure dominated moves further out, causing the surface layers of the disk to become more tenuous and highly ionized. The formation of a super-Eddington accretion disk in which much of the radiative energy is trapped in the accretion flow may also be relevant to quasars.

Curiously, one of the most luminous low redshift quasars, PDS 456, shows significant features at the iron-K energies (Reeves et al 2000). The features appear as a deep ionized edge and a possible broad line, and are modelled as either an ionized reflector (disk), or less likely as a strong highly ionized warm absorber.