25 years of XMM-Newton observations of the Sgr A complex

3D distribution and internal structure of the clouds

¹ Como Lake Center for Astrophysics (CLAP), DiSAT, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
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

^★ Corresponding author; giovanni.stel@inaf.it

Received: 3 July 2024
Accepted: 14 January 2025

Abstract

Context. Sgr A^⋆, the supermassive black hole at the center of the Milky Way, is currently very faint. However, X-ray radiation reflected by the Sgr A complex, a group of nearby molecular clouds, suggests that it went through one or more periods of high activity some hundreds of years ago.

Aims. We aim to determine whether previously proposed physical scenarios are consistent with the observed X-ray variability over the past 25 years. Furthermore, we seek to characterize the spatial distribution, shape, and internal structure of the clouds.

Methods. We exploited the whole set of available XMM-Newton observations of the Sgr A complex to date, extending the previously studied dataset on variability by at least 12 years. Starting from the recent Imaging X-ray Polarimetry Explorer result that places the so-called Bridge cloud 26 pc behind Sgr A^⋆, we reconstructed the line of sight (LOS) position of the remaining clouds in the molecular complex, assuming that they were illuminated by a single flare. Additionally, we derived the probability density function (PDF) of the molecular density. We also studied the 3D geometry of the complex in case two flares illuminate the clouds.

Results. As of spring 2024, the lightfront is still illuminating the Sgr A complex, with the Bridge currently being the brightest cloud. The other clouds in the complex have faded significantly. In the single flare scenario, the Sgr A complex is located ≃25 pc behind Sgr A^⋆. In the past 25 years, the illuminated region spans 10–15 pc along the LOS. The derived PDF of the molecular hydrogen exhibits a roughly log-normal distribution, consistent with previous Chandra results, with a potential excess at the high-density end.

Conclusions. Both a single flare and a multiple flares scenario can explain the observed X-ray variability. Previous concerns about the single flare scenario, raised by shorter monitoring, are now overcome in the 25 years of monitoring. If two flares illuminate the clouds, they must be separated by at least ∼30 years. We speculate that these clouds are closer to Sgr A^⋆ than the nuclear molecular ring at ≃100–200 pc and are possibly drifting from the ring to the inner region of the Galaxy.