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 center (GC) molecular clouds (MCs) within 15 arcmin from Sgr A*. We use XMM-Newton data (about 1.2 Ms of observation time) spanning about eight years. The MC spectra show all the features characteristic of reflection: (1) intense Fe K alpha, with equivalent width of about 0.7-1 keV, and the associated K beta line; (2) flat power-law continuum, and (3) a significant Fe K edge (tau similar to 0.1-0.3). The diffuse low ionization Fe K emission follows the MC distribution, nevertheless not all MC are Fe K emitters. The long baseline monitoring allows the characterization of the temporal evolution of the MC emission. A complex pattern of variations is shown by the different MCs, with some having constant Fe K emission, some increasing, and some decreasing. In particular, we observe an apparent superluminal motion of a light front illuminating a molecular nebula. This might be due to a source outside theMC (such as Sgr A* or a bright and long outburst of a X-ray binary), though 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, behavior similar to that 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 x 10(39) 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 10(36) 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*.