  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.
