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We examine the temporal and spectral properties of nine Fe-Ka brightmolecular clouds within about 30 pc of Sgr A*, in order to understand andconstrain the primary energising source of the Fe fluorescence. SignificantFe-Ka variability was detected, with a spatial and temporal pattern consistentwith that reported in previous studies. The main breakthrough that sets ourpaper apart from earlier contributions on this topic is the direct measurementof the column density and the Fe abundance of the MCs in our sample. We usedthe EW measurements to infer the average Fe abundance within the clouds to be1.6$\pm$0.1 times solar. The cloud column densities derived from the spectralanalysis were typically of the order of 10$^{23}$ cm$^{-2}$, which issignificantly higher than previous estimates. This in turn has a significantimpact on the inferred geometry and time delays within the cloud system. PastX-ray activity of Sgr A* is the most likely source of ionisation within themolecular clouds in the innermost 30 pc of the Galaxy. In this scenario, theX-ray luminosity required to excite these reflection nebulae is of the order of10$^{37}-10^{38}$ erg s$^{-1}$, significantly lower than that estimated for theSgr B2 molecular cloud. Moreover, the inferred Sgr A* lightcurve over the past150 years shows a long-term downwards trend punctuated by occasionalcounter-trend brightening episodes of at least 5 years duration. Finally, wefound that contributions to the Fe fluorescence by X-ray transient binaries andcosmic-ray bombardment are very likely, and suggest possible ways to study thislatter phenomenon in the near future.

The X-ray lightcurve of Sgr A* over the past 150 years inferred from Fe-Ka line reverberation in Galactic Centre molecular clouds