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, $ \sim$ 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_{\rm X}$$ =$ (1–3) $ \times$ 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.

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We present a new methodology to derive the positions of the Sgr B molecular clouds (MCs) along the line of sight, as an application study of the Galactic center diffuse X-rays (GCDX). The GCDX is composed of hot plasma emission of about 7 keV and 1 keV temperatures, and non-thermal continuum emission including the 6.4 keV line from neutral irons. The former, the Galactic center plasma emission (GCPE), is uniformly distributed over 1 degree in longitude, while the latter is clumpy emission produced by Thomson scattering and fluorescence from MCs irradiated by external X-rays (the X-ray reflection nebula emission: XRNE). We examined the Suzaku X-ray spectra of the GCPE and XRNE near to the Sgr B MC complex, and found that the spectra suffer from two different absorptions of N H (Abs1) ≥ 10 23 H cm −2 and N H (Abs2) ≃ 6×10 22 H cm −2 . Abs1 is proportional to the 6.4 keV-line flux, and hence is due to the MCs, while Abs2 is typical of interstellar absorption toward the Galactic center. Assuming that the GCPE plasma is spherically-extended around Sgr A* with a uniform density and the same angular distribution of the two temperature components, we quantitatively estimated the line-of-sight positions of the MCs from the flux ratio the GCPE spectrum suffered by Abs1 and that with no Abs1. The results suggest that the Sgr B MCs are located at the near side of Sgr A* in the GCPE.

Prominent K-shell emission lines of neutral iron (hereafter, Fe I-K lines) and hard-continuum X-rays from molecular clouds (MCs) in the Sagittarius B (Sgr B) region were found in two separate Suzaku observations in 2005 and 2009. The Xray flux of the Fe I-K lines decreased in correlation with the hard-continuum flux by a factor of 0.4-0.5 in four years, which is almost equal to the light-traveling across the MCs. The rapid and correlated time-variability, the equivalent width of the Fe I-K lines, and the K-edge absorption depth of Fe I are consistently explained by "X-ray echoes" due to the fluorescent and Thomson-scattering of an X-ray flare from an external source. The required flux of the X-ray flare depends on the distance to the MCs and its time duration. Even for a case with a minimum distance, the flux is larger than those of the brightest Galactic X-ray sources. Based on these facts, we conclude that the super-massive black hole Sgr A* exhibited a large-flare a few hundred years ago with a luminosity of more than 4 × 10 39 erg s −1 . The "X-ray echo" from Sgr B, located a few hundred light-years from Sgr A*, has now reached at the Earth.

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