New Evidence for High Activity of the Super-Massive Black Hole in our Galaxy

TL;DR

This study provides evidence that the super-massive black hole Sgr A* in our galaxy experienced a significant flare a few hundred years ago, as revealed by X-ray echoes from nearby molecular clouds.

Contribution

It presents new observational evidence of past high activity of Sgr A* through analysis of X-ray echoes in molecular clouds, confirming a major flare event.

Findings

Detection of FeI-K emission lines and hard X-ray continuum in Sgr B clouds.

Correlation between FeI-K flux decrease and continuum flux over four years.

Inference of a past giant flare from Sgr A* with luminosity >4x10^39 erg/s.

Abstract

Prominent K-shell emission lines of neutral iron (hereafter, FeI-K) and hard-continuum X-rays were found from molecular clouds (MCs) in the Sagittarius B (Sgr B) region with the two separate Suzaku observations in 2005 and 2009. The X-ray flux of FeI-K decreased in correlation to the hard-continuum flux by factor of 0.4-0.5 in 4 years, which is nearly equal to the light-travelling across the MCs. The rapid and correlated time-variability, the equivalent width of FeI-K, and the K-edge absorption depth of FeI are consistently explained by "X-ray echoes" due to the fluorescent and Thomson-scattering of an X-ray flare from an external source. The required flux of the X-ray flare depends on the distance to the MCs and the duration time. The flux, even in the minimum case, is larger than those of the brightest Galactic X-ray sources. Based on these facts, we conclude that the super-massive black hole, Sgr A*, exhibited a big-flare about a few hundred years ago and the luminosity of higher than 4x10^39 erg s^{-1}. The "X-ray echo" from Sgr B, located at a few hundred light-years from Sgr A*, now arrived at the Earth.