The iron line in MCG-6-30-15 has been observed to change flux
and profile on timescales of 
(Iwasawa et al. 1996,
1999). This is the shortest timescale on which detailed line
changes can currently be probed and there may indeed be line
variability on shorter timescales. MS99 note that such
variability is consistent with the line originating from a
Compton cloud of size 
.
However, in the Comptonization model, the continuum photons also pass through the same Comptonizing medium as the iron line photons. Thus, continuum variability can be used to place much tighter constraints on the size of the cloud. Any variability of the central source would be smeared out as the photons random walk through the cloud on a timescale of
Appreciable continuum variability in MCG-6-30-15 is observed on
timescales down to 
(Reynolds et al. 1995;
Yaqoob et al. 1997). Since we must have 
, an upper limit on the Compton cloud is 
, two orders of magnitude less than the size
assumed in MS99. Assuming a geometrically thick cloud and solar
abundances, the density of the material is 
.
In assessing the robustness of this constraint, it should be noted that the iron line in MCG-6-30-15 is always observed to be broad (although the width of the line does indeed vary, e.g. Iwasawa et al. 1996), and the source is always observed to vary its flux with a temporal power spectrum that extends down to 100s timescales (Lee et al. 1999b; Nowak & Chiang 1999; Reynolds 1999). Thus, it is difficult to support a model in which the Compton cloud is sometimes present (producing a broad line and a slowly varying continuum) and sometimes absent (producing a narrow line and a rapidly varying continuum).