Time Variability of the Neutral Iron Lines from the Sagittarius B2 Region and Its Implication of a Past Outburst of Sagittarius A

Abstract

We investigated long-term X-ray behavior from the Sgr B2 complex using archival data of the X-ray satellites Suzaku, XMM-Newton, Chandra, and ASCA. The observed region of the Sgr B2 complex included two prominent spots in the FeI Kα line at 6.40keV, a giant molecular cloud, M0.66-0.02, known as the ``Sgr B2 cloud'' and an unusual X-ray source, G0.570-0.018. Although these 6.40keV spots have spatial extensions of a few parsec scale, the morphology and flux of the 6.40keV line has been time variable for 10 years, in contrast to the constant flux of the FeXXV Kα line at 6.67keV in the Galactic diffuse X-ray emission. This time variation is mostly due to M0.66-0.02; the 6.40keV line flux declined in 2001, and decreased to 60% in the time span 1994--2005. The other spot, G0.570-0.018, is found to be conspicuous only in the Chandra observation in 2000. From the long-term time variability (∼10 years) of the Sgr B2 complex, we infer that the Galactic Center black hole Sgr A^* was X-ray bright in the past 300 years, and exhibited a time variability with a period of a few years.