Iron K-alpha echoes from the circumnuclear disk orbiting Sgr A*

TL;DR

This study analyzes Fe K-alpha fluorescence signals from the circumnuclear disk around Sgr A* using 20 years of X-ray data, revealing properties of the disk and illuminating past energetic events in the Galactic center.

Contribution

It provides the first detailed analysis of Fe K-alpha echoes from the CND, constraining its physical properties and linking observed signals to past outbursts of SGR J1745-2900.

Findings

Detected significant Fe K-alpha line excess in the eastern CND region.

Estimated the hydrogen column density of the CND to be approximately 10^{23} cm^{-2}.

Linked the fluorescence to the 2013 outburst of magnetar SGR J1745-2900.

Abstract

Molecular clouds in the Galactic center (GC) reprocess radiation from past outbursts of nearby high-energy sources, generating a bright Fe K-alpha fluorescence at 6.4 keV. The closest clouds to the GC are only $\simeq 1.5$ pc from Sgr A*, forming a torus-like structure known as the circumnuclear disk (CND). The study of fluorescence emission can lead to a characterization of the illuminating source(s), the reflecting clouds, and the global geometry of such a system lying in the GC. The primary purpose of our study is to analyze possible fluorescence signals arising in the CND. This signal would allow us to constrain the CND's physical properties and the source-reflector system's geometry. By exploiting the last $\simeq 20$ years of XMM-Newton observations of the GC, we studied the variability of the Fe K-alpha line in the region around Sgr A*. We identified regions with a flux excess and computed the spectrum therein. We then derived the hydrogen column density of the CND after relating the intensity of the 6.4 keV line to the total energy emitted by known transient sources in the region. Starting from data collected in 2019, we find significant line excesses in a region compatible with the eastern portion of the CND. The echo radiation can be linked to the 2013 outburst of the magnetar SGR J1745-2900. We derive a mean effective hydrogen column density of the CND in the eastern region of $\simeq 10^{23}$ cm$^{-2}$. The scenario depicted is physically plausible, given the luminosity, the position of the illuminating source, and the expected density of the CND.