Twenty-year-long X-ray monitoring of the Galactic Center: echoes of past Sgr A* outbursts

Sgr A* is the least active supermassive black hole known to us. Whether it has more substantial increases in activity as observed in low-luminosity Active Galactic Nuclei is still under discussion. Indication of such past activity of Sgr A* has come from Galactic center molecular clouds. X-ray reflection signals from molecular clouds in the central molecular zone can help us reconstruct Sgr A* activity history in the past few hundred years. In this talk I am going to discuss our newest results X-ray variability of two Galactic center molecular clouds, Sgr B2 and Bridge, on a 20-year-long baseline. Using archival and newly acquired NuSTAR and XMM-Newton observations, we found that Sgr B2 has been dimming down in the past two decades, while Bridge has been consistently brightening up and rising to the brightest diffuse X-ray feature in the Galactic center. These two molecular clouds provides valuable samples for us to study the brightening and decaying stage of a Sgr A* illumination event, as well as put tight constraints on Sgr A* luminosity 100-400 years ago.