  IGR J16318-4848 is the prototype and one of the more extreme examples of the
new class of highly obscured Galactic X-ray sources discovered by INTEGRAL. A
monitoring campaign on this source has been carried out by XMM-Newton and
INTEGRAL, consisting in three simultaneous observations performed in February,
March and August 2004. The long-term variability of the Compton-thick
absorption and emission line complexes will be used to probe the properties of
the circumstellar matter. A detailed timing and spectral analysis of the three
observations is performed, along with the reanalysis of the XMM-Newton
observation performed in February 2003. The results are compared with
predictions from numerical radiative transfer simulations to derive the
parameters of the circumstellar matter. Despite the large flux dynamic range
observed (almost a factor 3 between observations performed a few months apart),
the source remained bright (suggesting it is a persistent source) and
Compton-thick (NH >1.2x10^24 cm-2). Large Equivalent Width (EW) emission lines
from Fe Kalpha, Fe Kbeta and Ni Kalpha were present in all spectra. The
addition of a Fe Kalpha Compton Shoulder improves the fits, especially in the
2004 observations. Sporadic occurrences of rapid X-ray flux risings were
observed in three of the four observations. The Fe Kalpha light curve followed
the continuum almost instantaneously, suggesting that the emission lines are
produced by illumination of small-scale optically-thick matter around the
high-energy continuum source. Using the iron line EW and Compton Shoulder as
diagnostic of the geometry of the matter, we suggest that the obscuring matter
is in a flattened configuration seen almost edge-on.
