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

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 K$\alpha$ line at 6.40 keV, a giant molecular cloud, M0.66$-$0.02, known as the “Sgr B2 cloud” and an unusual X-ray source, G0.570$-$0.018. Although these 6.40 keV spots have spatial extensions of a few parsec scale, the morphology and flux of the 6.40 keV line has been time variable for 10 years, in contrast to the constant flux of the Fe XXV K$\alpha$ line at 6.67 keV in the Galactic diffuse X-ray emission. This time variation is mostly due to M0.66$-$0.02; the 6.40 keV line flux declined in 2001, and decreased to 60% in the time span 1994–2005. The other spot, G0.570$-$0.018, is found to be conspicuous only in the Chandra observation in 2000. From the long-term time variability ($\sim$10 years) of the Sgr B2 complex, we infer that the Galactic Center black hole Sgr A$^\ast$ was X-ray bright in the past 300 years, and exhibited a time variability with a period of a few years.