A Time-Variable X-Ray Echo: Indications of a Past Flare of the Galactic-Center Black Hole

A time-variability study of the neutral iron line flux at 6.40 keV in the Sgr B2 region from data of Suzaku and Chandra is presented. The highly ionized iron line at 6.68 keV is due to Galactic Center Diffuse X-rays (GCDX), and is thus time invariable. By comparing the 6.68 keV and 6.40 keV line fluxes, we found that the 6.40 keV flux from the Sgr B2 complex region is time variable; particularly the giant molecular cloud M 0.66−0.02, known as “Sgr B2 cloud” is highly variable. The variability of the 6.40 keV line in intensity and spatial distribution strongly supports the scenario that the molecular clouds in the Sgr B2 region are X-ray Reflection Nebulae irradiated by the Galactic Center (GC) black hole Sgr A.