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
Kalpha, with EW of about 0.7-1 keV, and the associated Kbeta line; ii) flat
power law continuum and iii) a significant Fe K edge (tau~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 1.5~10^39 erg/s) 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 MC places an upper limit of 10^36 erg/s 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*.
