Time Variability of the Neutral Iron Lines from the Sgr B2 Region and its Implication of a Past Outburst of Sgr A$^*$

TL;DR

This study analyzes long-term X-ray data from multiple satellites to reveal that the Sgr B2 region's iron line emissions vary over a decade, implying Sgr A* was more active in the past 300 years.

Contribution

It provides the first long-term variability analysis of the Sgr B2 iron lines, linking past activity of Sgr A* to observed X-ray features.

Findings

The 6.40 keV line flux decreased to 60% over 10 years.

Sgr A* was X-ray bright approximately 300 years ago.

The variability suggests episodic activity of the Galactic Center black hole.

Abstract

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-$\alpha$ line at 6.40 keV, a giant molecular cloud M 0.66$-$0.02 known as the ``Sgr B2 cloud'' and an unusual X-ray source G 0.570$-$0.018. Although these 6.40 keV spots have spatial extensions of a few pc scale, the morphology and flux of the 6.40 keV line has been time variable for 10 years, in contrast to the constant flux of the Fe XXV-K$\alpha$ line at 6.67 keV in the Galactic diffuse X-ray emission. This time variation is mostly due to M 0.66$-$0.02; the 6.40 keV line flux declined in 2001 and decreased to 60% in the time span 1994--2005. The other spot G 0.570$-$0.018 is found to be conspicuous only in the Chandra observation in 2000. From the long-term time variability ($\sim$10 years) of the Sgr B2 complex, we infer that the Galactic Center black hole Sgr A$^*$ was X-ray bright in the past 300 year and exhibited a time variability with a period of a few years.