  Using CCD detectors onboard the forthcoming X-ray observatories Chandra and
XMM, it is possible to devise a measurement of the absolute density of heavy
elements in the hypothetical warm gas filling intercluster space. This gas may
be the largest reservoir of baryonic matter in the Universe, but even its
existence has not been proven observationally at low redshifts. The proposed
measurement would make use of a unique filament of galaxy clusters spanning
over 700 Mpc (0.1<z<0.2) along the line of sight in a small area of the sky in
Aquarius. The surface density of Abell clusters there is more than 6 times the
sky average. It is likely that the intercluster matter column density is
enhanced by a similar factor, making its detection feasible under certain
optimistic assumptions about its density and elemental abundances. One can
compare photoabsorption depth, mostly in the partially ionized oxygen edges, in
the spectra of clusters at different distances along the filament, looking for
a systematic increase of depth with the distance. The absorption can be
measured by the same detector and through the same Galactic column, hence the
differential test. A CCD moderate energy resolution (about 100 eV) is adequate
for detecting an absorption edge at a known redshift.
