Traces of past activity in the Galactic Centre

TL;DR

This paper reviews the historical activity of the supermassive black hole Sgr A* in the Galactic Center, highlighting evidence of past active phases and their impact on surrounding structures like the CMZ.

Contribution

It synthesizes current understanding of X-ray signatures of past activity and presents detailed imaging of the AGN torus in the Galactic Center.

Findings

Evidence of past active periods of Sgr A* in surrounding medium

The CMZ structure remains unchanged since last active phase

Imaging of the AGN torus in the Galactic Center

Abstract

The Milky Way centre hosts a supermassive Black Hole (BH) with a mass of ~4*10^6 M_Sun. Sgr A*, its electromagnetic counterpart, currently appears as an extremely weak source with a luminosity L~10^-9 L_Edd. The lowest known Eddington ratio BH. However, it was not always so; traces of "glorious" active periods can be found in the surrounding medium. We review here our current view of the X-ray emission from the Galactic Center (GC) and its environment, and the expected signatures (e.g. X-ray reflection) of a past flare. We discuss the history of Sgr A*'s past activity and its impact on the surrounding medium. The structure of the Central Molecular Zone (CMZ) has not changed significantly since the last active phase of Sgr A*. This relic torus provides us with the opportunity to image the structure of an AGN torus in exquisite detail.