  Observations of the diffuse emission in the 8--22 keV energy range, elongated
parallel to the Galactic plane (Sunyaev et al. 1993) and detection of the
strong 6.4 keV fluorescent line with $\sim$ 1 keV equivalent width from some
giant molecular clouds (e.g. Sgr B2) in the Galactic Centre region (Koyama
1994) suggest that the neutral matter of these clouds is (or was) illuminated
by powerful X-ray radiation, which gave rise to the reprocessed radiation. The
source of this radiation remains unknown. Transient source close to the Sgr B2
cloud or short outburst of the X-ray emission from supermassive black hole at
the Galactic Centre are the two prime candidates under consideration. We argue
that new generation of X-ray telescopes combining very high sensitivity and
excellent energy and angular resolutions would be able to discriminate between
these two possibilities studying time dependent changes of the morphology of
the surface brightness distribution, the equivalent width and the shape of the
fluorescent line in the Sgr B2 and other molecular clouds in the region. We
note also that detection of broad and complex structures near the 6.4 keV line
in the spectra of distant AGNs, which are X-ray weak now, may prove the
presence of violent activity of the central engines of these objects in the
past. Accurate measurements of the line shape may provide an information on the
time elapsed since the outburst. Proper motion (super or subluminal) of the
fluorescent radiation wave front can give additional information on the
location of the source. Observations of the described effects can provide
unique information on the matter distribution inside Sgr B2 and other giant
molecular clouds.
