2011/09/08 by Masayoshi Nobukawa, S. G. Ryu, Syukyo G. Ryu +3 · 2 citations
Physics and Astronomy · #Astrophysical Phenomena and Observations #astro-ph.HE
paper · pdf · doi:10.1088/2041-8205/739/2/l52
4 pages, 3 figures, accepted for publication in ApJL
openalex publication_date 2011/09/08 · arxiv created 2011/09/09 · arxiv updated 2015/05/29 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
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 X-ray 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 supermassive 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 the Earth.