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

TL;DR

The paper tackles isolating the intrinsic non-thermal X-ray emission of the Arches star cluster from surrounding molecular-cloud reflection in the Galactic Center. It analyzes deep XMM-Newton data from 2020 and Chandra data from 2022 to perform spatially resolved spectral analysis, modeling the emission with a thermal plasma plus a hard power-law. The results show no significant Fe Kα 6.4 keV reflection within the cluster core, and the spectrum comprises a thermal component around $kT \approx 2$ keV and a hard non-thermal component with $\Gamma \approx 1.5$, with the hard X-ray flux dominated by the non-thermal part and localized to the core. These findings reinforce the view of compact stellar clusters as Galactic cosmic-ray accelerators and motivate future high-energy observations to probe particle acceleration mechanisms in the Galactic Center, particularly above ~10 keV.

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.

Figures (7)

• Figure 1: Upper panels: XMM-Newton/EPIC exposure$-$ and background$-$corrected maps of the Arches cluster region containing the line (and continuum) emission at 6.4 keV ( left panels) and 6.7 keV ( right panels) observed in 2020. The images, initially corrected for particle background, were corrected for astrophysical background measured in the green dashed annulus. Bottom panels: sky images, additionally corrected for continuum emission, represent Fe K$\alpha$ emission line maps at 6.4 keV and 6.7 keV. The green circle with radius of $15"$ indicates the region used to characterize the Arches cluster X-ray emission, which shows strong Fe K$\alpha$ emission at 6.7 keV. Yellow ellipse denotes reference region of the Arches molecular cloud observed earlier at 6.4 keV, adopted from T12. Green dashed annulus is also used in spectral analysis to extract background. All sky images were convolved with a Gaussian ($\sigma=1.5$ pix, the pixel size is $4"$).
• Figure 2: Merged MOS1, MOS2 and pn X-ray spectra of the Arches cluster between 4 and 10 keV for three XMM-Newton observations in 2020. Best-fitting model (solid black line) is composed by a combination of an absorbed power-law (dashed blue) normalized in energy range shown by vertical blue dashed lines, and two Gaussian functions attributed to Fe K$\alpha$ lines at 6.4 keV (dark green) and 6.7 keV (red).
• Figure 3: Chandra X-ray spectra of the Arches cluster between 4 and 10 keV for three epochs in 2022. See Fig. \ref{['fig:lines:spe:xmm']} for reference.
• Figure 4: Merged MOS1, MOS2 and pn X-ray spectra of the Arches cluster between 2 and 10 keV for three XMM-Newton observations in 2020. Best-fitting model (solid black line) is composed by a combination of an absorbed apec (blue) and power-law (red).
• Figure 5: Chandra 2022 X-ray spectra of the Arches cluster between 1 and 9 keV for three epochs in 2022. Best-fitting model (solid black line) is composed by a combination of an absorbed apec (blue) and power-law (red). Green dashed lines show positions of $K\alpha$ lines of He-like Si at ${\sim}1.85$ keV, S at ${\sim}2.45$ keV and Fe at $6.4$ keV (fixed), respectively, added to the fitting procedure (see Table \ref{['tab:spec:apec']}).