Direct Detection of the Non-thermal X-ray Emission from the Arches Star Cluster

Abstract

The compact stellar clusters have emerged as particularly promising candidates for cosmic rays (CR) accelerators. The star clusters, recently observed in \(\gamma\)-rays, are also known sources of non-thermal X-ray emission, which is due to synchrotron or inverse-Compton scattering of relativistic electrons. Thus, the search for the non-thermal X-ray emission from stellar clusters is of particular interest. Until recent time the X-ray emission of the Arches star cluster in the Galactic Center was mixed with non-thermal emission of the surrounding molecular cloud, associated with reflection of hard X-ray irradiation. This reflected emission has been observed to fade, giving us a chance to characterize intrinsic non-thermal emission of the Arches cluster. In this work we demonstrate that Fe \(K\alpha\) line emission at 6.4 keV, attributed to the reflected non-thermal emission of the molecular cloud in 2000–2010, is not detected in deep observations with XMM–Newton in 2020 and Chandra in 2022, leaving stellar cluster well isolated. We showed that the Arches non-thermal emission is localized in the cluster’s core and characterized by a relatively weak, hard (\(\Gamma\approx 1.5\)) power-law spectral continuum with 2–10 keV flux of \({\sim}10^{-13}\) erg s\({}^{-1}\) cm\({}^{-2}\).