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As outlined above, at least 85 per cent of accretion power is absorbed. Since about ten per cent is in quasars which show very little absorption, this means that most lines of sight out of the remaining objects are highly absorbed. This is difficult for the standard obscuring torus model, which could absorb perhaps one half to two thirds of all sight lines. Even then it is unclear what inflates the torus, which is supposed to be cold and molecular. Dissipation in in a system of orbiting clouds should cause it to flatten into a disc, with lowcovering factor.

Energy must be continuously injected into any cold absorbing cloud system to keep it inflated and so sky covering. One plausible solution is that a gas-rich star cluster surrounds the black hole and it is the massive stars (winds and supernovae) which supply the energy (Fabian et al 1998). The surrounding starburst can thereby obscure the active nucleus.

The starburst should enhance the metallicity of the absorbing gas. This makes a given mass of gas more efficient at absorbing X-rays and indeed increases the effect of absorption before Compton down-scattering comes into play. This is important in opening up the parameter space for model-fitting of the XRB spectrum (Wilman & Fabian 1999). Fuelling of the nucleus is an old problem (see e.g. Shlosman et al 1990). Although there may be lots of gas around the nucleus, angular momentum may prevent it from rapidly accreting to the centre. In this respect, a hot phase in the surrounding medium may be important, with Bondi accretion from this phase being the dominant mechanism (see e.g. Nulsen & Fabian 1999). Angular momentum may be transported outward by turbulence within such a hot phase, so allowing rapid accretion to proceed.