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Reflected X‐ray Emissions on Giant Molecular Clouds — Evidence of the Past Activities of Sgr A*
Author(s) -
Murakami Hiroshi,
Senda Atsushi,
Maeda Yoshitomo,
Koyama Katsuji
Publication year - 2003
Publication title -
astronomische nachrichten
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.394
H-Index - 63
eISSN - 1521-3994
pISSN - 0004-6337
DOI - 10.1002/asna.200385075
Subject(s) - physics , galactic center , astrophysics , molecular cloud , reflection nebula , astronomy , luminosity , line (geometry) , spectral line , nebula , interstellar medium , galaxy , stars , geometry , mathematics
We have found strong 6.4‐keV line emissions from the giant molecular clouds in the Galactic center region: Sgr B2, Sgr C, and M0.11–0.08 (at the Radio Arc region). The high angular resolution of Chandra reveals that the 6.4‐keV line emissions are indeed coincident with the clouds, and shifted towards the Galactic center. The X‐ray spectra have very strong 6.4‐keV lines with equivalent widths ≥1 keV and are attenuated by larger column densities of interstellar gas. These characteristics imply that the massive molecular clouds are irradiated by an external X‐ray source in the direction of the Galactic center and emit fluorescent and scattered X‐rays. These clouds are new category of X‐ray source: “X‐ray Reflection Nebula”. The reflected X‐ray flux reveals the recent luminosity history of the primary irradiating source, which may be the massive black hole Sgr A*, according to the light travel time to each cloud. Making use of the radio determinations of the cloud masses, we find that Sgr A* was as luminous as 10 39 erg s −1 a few hundred years ago, and has gradually decreased to present value.