Context : 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 .

Aims : The long-term variability of the Compton-thick absorption and emission line complexes will be used to probe the properties of the circumstellar matter .

Methods : 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 .

Results : 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 ( N _ { H } > 1.2 \times 10 ^ { 24 } cm ^ { -2 } ) .
Large Equivalent Width ( EW ) emission lines from Fe K _ { \alpha } , Fe K _ { \beta } and Ni K _ { \alpha } were present in all spectra .
The addition of a Fe K _ { \alpha } 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 K _ { \alpha } 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 .

Conclusions :