5.5. Optically Thin Emission and Multiphase Flows
The problems with accretion disk models have led some authors to propose optically thin emission as a possible explanation for the BBB. The simplest model is simply free-free emission in an optically thin plasma (see, e.g., Barvainis 1993), possibly powered by reprocessing of X-rays or by direct mechanical heating. Reprocessing of X-rays into BBB photons by cool, optically thin clouds has been considered by many authors (Ferland & Rees 1988; Ferland, Korista, & Peterson 1990; Celotti, Fabian, & Rees 1992; Collin-Souffrin et al. 1996; Kuncic, Celotti, & Rees 1997). Optically thin emission is of course also accompanied by substantial line and edge features, but just as in disks (see, e.g., Kriss 1994), Comptonization and relativistic smearing might help to reduce them in the observed SED.
The inner regions of accretion disks themselves may be very inhomogeneous and exist in a multiphase flow consisting of clouds in a hot, magnetized intercloud medium. Krolik (1998) has proposed that such an equilibrium would be both thermally and viscously stable, in contrast to the standard radiation pressure-supported thin disk with the usual -viscosity. Gammie (1998) has found that magnetized, radiation pressure supported disks are likely to suffer from a dynamical photon bubble instability, which might in fact lead to just such a multiphase equilibrium.
As we noted in Section 4.1, the real problem facing accretion disk models may be in explaining the origin of the ionizing continuum. In addition to the usual disk and hot, Comptonizing corona, Magdziarz et al. (1998) have invoked a third phase to explain the EUV and soft X-rays in NGC 5548: a warm (~ 0.3 keV), optically thick Comptonizing medium (see Fig. 24). This component is reminiscent of models of the soft X-ray excess that invoke thermal Comptonization in the inner regions of the accretion disk itself (Czerny & Elvis 1987; Maraschi & Molendi 1990; Ross et al. 1992; Shimura & Takahara 1993, 1995; Dörrer et al. 1996). Note that Zheng et al.'s (1997) interpretation of the EUV/soft X-ray component in the HST composite was also based on Comptonization but in a hotter, more optically thin medium. The geometry of the three components is very unclear, however. A thermal Comptonization origin would be one way of getting rid of the Lyman edge in this Seyfert galaxy. In addition, it provides another interpretation of the fact that the optical/UV component becomes harder when brighter - here the disk component could be constant, while the soft excess component becomes brighter. Much more work needs to be done on theoretical models of such multiphase flows.
Figure 24. Broadband average spectrum of NGC 5548, from simultaneous archival optical / IUE / Ginga data, as well as nonsimultaneous ROSAT and GRO / OSSE data. The upper panel shows the data and fitted model, while the lower panel shows a decomposition of the spectrum into disk and thermal Comptonization continuum components. The dot-short-dashed curve shows the disk, modeled using blackbodies. The short-dashed curve is the soft excess component, modeled with thermal Comptonization in an optically thick, warm medium. This component dominates the spectrum from the optical/UV to 1 keV. The long-dashed and dotted curves show the hot thermal Comptonization and reflection components, respectively. The dot-long-dashed curve shows the disk component fitted to the optical / IUE data separately and is analogous to the cool disk fit to the composite spectrum shown in Fig. 17 (from Magdziarz et al. 1998).