Time Variability of the Neutral Iron Lines from the Sgr B 2 Region and its Implication of a Past Outburst of Sgr A ∗

We investigate long-term X-ray behaviors 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 includes two prominent spots in the Fe I K-α 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 pc 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 FeXXV-Kα 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 (∼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 year and exhibited a time variability with a period of a few years.