6. Observational appearance

Theoretical models predict both line and continuous electromagnetic spectra of accretion discs. Comparison with observations shows a rather good general agreement. However, several details are not satisfactory fitted and others are fixed by empirical ad hoc assumptions. Generally, the radiative transfer through the disc is not treated as accurately as in the case of stellar atmospheres. The two most acute difficulties that one faces in the accretion disc case are: (a) the disc geometry calls for a 3-D treatment of the problem, (b) the viscous energy generation rate is not known as a function of the position in the disc. Fundamental papers that discuss methods to deal with these difficulties: ????; see also ????. In the black hole and neutron star accretion discs, the fact that radiation moves in a highly curved space (spacetime) further complicates the problem of calculating the spectra. Several "ray tracing" methods of calculating have been developed to deal with this issue; see ???? and ????. Jean-Alain Marck was a pioneer of the modern methods in the ray tracing.

The protoplanetary discs appear in the spectra of young stellar objects as infra-red excess on top of a stellar blackbody signature. Depending on the stage of the system, the excess hump can dominate (class I) the emission profile or is barely notable (class III). Protoplanetary discs contain a large amount of dust, e.g., silicates, minerals or water, whose chemical composition and geometrical shape show up as a forest of line features in the spectrum and greatly influence the opacity and evolution of the disc (Dullemond & Dominik, 2005).

The Cataclysmic Variables (CVs) with accretion discs (i.e., the WD is not or only weakly magnetised) can be of very different types. The classical or recurring novae discs show a stupendous variation of luminosity during the outburst phase from hundreds to billions times the original disc luminosity (the larger the outburst amplitude, the faster the nova fades). Moreover, collimated jets may emanate from their central regions. The dwarf novae discs are controlled by a temporary enhancement of the rate of mass transfer. The dust or debris discs resulting from tidally disrupted asteroids are usually cold and underluminous. Generally, discs of CVs are divided into two states:

• the high state (high luminosity, active mass-transfer)
• the low state (low luminosity, quiescent)

The X-ray Galactic binaries (BHBs) display a number of distinct spectral states (Zdziarski & Gierlinski 2004; Remillard & McClintock, 2006; Done et al., 2007):

• the very high state (very luminous)
• the thermal state (disc dominated, luminous, high/soft state),
• the intermediate state
• the power-law state (slightly less luminous, low/hard state with strong variability) ,
• the quiescent state (very underluminous).

The thermal state is well described by the thin disc model and a multi-colour disc (MCD) blackbody model (e.g., diskbb, Mitsuda et al. 1984; Zimmerman et al. 2005; available in XSPEC, Arnaud et al. 1996). Recently, fully relativistic versions of the MCD model for arbitrary BH spin (kerrbb, Li et al. 2005; bhspec, Davis & Hubeny 2006) have been developed, based on the relativistic thin disc model of Novikov & Thorne (1973). These models provide excellent fits to the X-ray spectra of BHBs in the thermal and intermediate state.

The active galactic nuclei discs (AGN)

The gamma ray burst discs (GRBs)