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The physical conditions of the gas forming the narrow line regions (NLR) inactive galactic nuclei (AGN) have been extensively studied in the optical band.We take advantage of the spectacular extension ($\sim$15") of the NLR in thetype II Seyfert galaxy NGC 5252 and of the complementary characteristics of$XMM$--$Newton$ and $Chandra$ to investigate the physical conditions of the gasin this galaxy. The X-ray data from $XMM$--$Newton$ are used to define thespectral properties of the ionising nuclear source. The $Chandra$ data are usedto trace the spatial characteristics of the soft X-ray emission. Thisinformation is then compared to the HST characteristics. The X-ray spectrum ofthe nucleus of NGC 5252 is intrinsically flat and absorbed by neutral gas witha column density N~10$^{22}$ cm$^{-2}$. Below 1 keV a soft excess is detected.The high-resolution spectrum obtained with the XMM-Newton RGS shows thepresence, in the 0.2-1.5 keV range, of emission lines which strongly indicatethat the soft X-ray component is due to ionised gas. Moreover, the soft X-rayemission is spatially resolved around and well overlaps the images obtained innarrow [OIII] optical band. The [OIII]/soft-X flux ratios along the ionisationcones is basically constant. This indicates that the electron density does notsignificantly deviates from the r$^{-2}$ law (constant ionisation parameter).This result combined with previous optical studies suggest two plausible butdifferent scenarios in the reconstruction of the last 30000 years history ofthe central AGN. The most promising one is that the source is indeed a "quasarrelic" with steady and inefficient energy release from the accretion of matteronto the central super-massive black-hole. This scenario is suggested also bythe flat nuclear X-ray spectrum that suggests an advection dominate accretionflow (ADAF) like emission mechanism.

X-ray imaging of the ionisation cones in NGC 5252