Preprint typeset using LATEX style emulateapj v. 11/10/09 DISCOVERY OF A SUPERLUMINAL FE K ECHO AT THE GALACTIC CENTER: THE GLORIOUS PAST OF SGR A * PRESERVED BY MOLECULAR CLOUDS

We present the result of a study of the X-ray emission from the Galactic Centre (GC) Molecular Clouds (MC) within 15 arcmin from Sgr A*. We use XMM-Newton data (about 1.2 Ms of observation time) spanning about 8 years. The MC spectra show all the features characteristic of reflection: i) intense Fe Kα, with EW of about 0.7-1 keV, and the associated Kβ line; ii) flat power law continuum and iii) a significant Fe K edge (τ ∼ 0.1 − 0.3). The diffuse low ionisation Fe K emission follows the MC distribution, nevertheless not all MC are Fe K emitters. The long baseline monitoring allows the characterisation of the temporal evolution of the MC emission. A complex pattern of variations is shown by the different MC, with some having constant Fe K emission, some increasing and some decreasing. In particular, we observe an apparent super-luminal motion of a light front illuminating a Molecular nebula. This might be due to a source outside the MC (such as Sgr A * or a bright and long outburst of a X-ray binary), while it cannot be due to low energy cosmic rays or a source located inside the cloud. We also observe a decrease of the X-ray emission from G0.11-0.11, behaviour similar to the one of Sgr B2. The line intensities, clouds dimensions, columns densities and positions with respect to Sgr A*, are consistent with being produced by the same Sgr A * flare. The required high luminosity (about 1.5×1039 erg s−1) can hardly be produced by a binary system, while it is in agreement with a flare of Sgr A * fading about 100 years ago. The low intensity of the Fe K emission coming from the 50 and the 20 km s−1 MC places an upper limit of 1036 erg s−1 to the mean luminosity of Sgr A * in the last 60-90 years. The Fe K emission and variations from these MC might have been produced by a single flare of Sgr A*