Light curves of the 6.4 keV line emission obtained by fitting the phenomenological model and integrated over the regions Sgr C1 (left), Sgr C2 (centre) and Sgr C4 (right). The black circles and the red squares correspond to XMM-Newton and Chandra observations, respectively. The grey shaded bars show the average level of background emission measured in the control region (their thickness represents one standard deviation).