Direct detection of the non-thermal X-ray emission from the Arches star cluster

TL;DR

This study isolates and characterizes the intrinsic non-thermal X-ray emission from the Arches star cluster, demonstrating it is localized in the core with a hard spectrum, distinct from surrounding molecular cloud reflection.

Contribution

It provides the first clear detection and characterization of the non-thermal X-ray emission originating directly from the Arches star cluster, separate from reflected emission.

Findings

Fe K_alpha line emission from the molecular cloud is not detected in recent observations.

The non-thermal emission is localized in the cluster core.

The spectrum is hard with a photon index of approximately 1.5.

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~1.5) power-law spectral continuum with 2-10 keV flux of ~10E-13 ergs/s/cm^2.