X-Ray Echo from the Sagittarius C Complex and 500-year Activity History of Sagittarius A

This paper presents Suzaku results obtained for the Sagittarius (Sgr) C region using the concept of X-ray reflection nebulae (XRNe) as the echo of past flares from a super-massive black hole, Sgr A*. The Sgr C complex is composed of several molecular clouds proximately located in projected distance. The X-ray spectra of Sgr C were analyzed based on the view that XRNe are located inside the Galactic center plasma X-ray emission with an oval distribution around Sgr A*. We found that XRNe are largely separated in the line-of-sight position, and are associated with molecular clouds in different velocity ranges detected by radio observations. We also applied the same analysis to Sgr B XRNe, and completed a long-term light curve for Sgr A* occurring in the past. As a new finding, we determined that Sgr A* has already experienced periods of high luminosity, similar to 500 years ago, which is longer than the previously reported value. Our results are consistent with a scenario that Sgr A* was continuously active with sporadic flux variabilities of L-x = (1-3) x 10(39) erg s(-1) in the past 50 to 500 years. The average past luminosity was approximately 4-6 orders of magnitude higher than that presently observed. In addition, two short-term flares of 5-10 years have been found. Thus, the past X-ray flare should not be a single short-term flare, but can be interpreted as multiple flares superposed on a long-term high state.