Dr Renzo Capelli (MPE Garching)
The X-ray reflection nebulae (XRN) in the Galactic Centre (GC) region have been proposed to be the smoking gun of a past low AGN activity of Sgr A*, which is suspected to have undergone a flare about 10^4 times brighter than the brightest flare ever measured, this high state happening some hundred years ago. This enhanced X-ray activity must have left a trace in the diffuse emission permeating the GC region, especially in the ionization of molecular clouds (MCs) in the central molecular zone (CMZ) seen through the Fe fluorescent line at 6.4 keV, and the Thomson scattering of hard X-rays into the line of sight by the MCs. The picture is however still unclear and confusing, since other high energetic phenomena can account for the 6.4 keV emission from MCs in the CMZ. In my talk I will review the history and the main discoveries related to this topic, and will show that the XRN/Sgr A* scenario alone has strong difficulties in accounting for the observed fluorescence; indeed, a strong component induced by particle (subrelativistic electrons and/or protons) interaction with the MCs is needed in order to account for all the spectral/temporal properties of the observed MCs. I will also show the results of my studies on the Fe Ka emission from the MCs in the Arches cluster region, which state that Sgr A* cannot be the source of ionization for every 6.4 keV bright cloud; these MCs are to be considered the prototype for particle bombardment induced Fe Ka line emission. I will also present the recent discovery of the MC showing the fastest variability in the 6.4 keV line flux, clearly related to an X-ray transient source other than Sgr A*.