Iron Kα echoes from the circumnuclear disk orbiting Sɡr A^⋆★

¹ DiSAT, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
e-mail: giovanni.stel@inaf.it
² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁴ INFN, Sezione Milano-Bicocca, P.za della Scienza 3, 20126 Milano, Italy

Received: 14 June 2023
Accepted: 7 August 2023

Abstract

Context. Molecular clouds in the Galactic center (GC) reprocess radiation from past outbursts of nearby high-energy sources, generating a bright Fe Kα fluorescence at 6.4 keV. The closest clouds to the GC are only ≃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.

Aims. 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.

Methods. By exploiting the last ≃20 yr of XMM-Newton observations of the GC, we studied the variability of the Fe Kα 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.

Results. 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 ≃10²³ cm⁻².

Conclusions. The scenario depicted is physically plausible, given the luminosity, the position of the illuminating source, and the expected density of the CND. Further observations could link the variability of the echo signal to the light curve of the illuminating source. In this way, it would be possible to characterize the cloud response to the radiation front, achieving a more accurate estimate of the cloud parameters.