Abstract :
[en] Context. Westerlund 1 (Wd 1) is the closest supermassive star cluster to the Sun, containing more than 100 000 stars of all spectral types and masses down to brown dwarfs. This population considerably heats the surrounding interstellar medium (ISM), making Wd 1 a relevant place to investigate stellar feedback on intracluster gas. Aims. We present the most detailed X-ray study to date of the diffuse emission in this region, aimed at providing the most favourable observational configuration for distinguishing and quantifying the point-source contribution to the true diffuse emission. Methods. We analysed 36 Chandra ACIS-I observations of Wd 1 within an 8 × 8 arcmin window. After removing 4922 point sources with ACIS Extract, using energy-dependent point spread function (PSF) models and adaptive 99% enclosed-energy masks, we subtracted PSF-wing contamination from overlapping neighbours. It is critical in this crowded field to correct for background. We applied adaptive smoothing to reveal hot gas between stars in the soft (0.5-1.2 keV), medium (1.2-1.9 keV), and hard (1.9-7.0 keV) bands. Results. The spectral fitting suggests a shocked thermal plasma, with soft emission extending beyond the stellar core and diffuse X-rays correlating spatially with the massive stars. The cluster core (region #2) is best described by an APEC+PSHOCK model, capturing the continuous range of post-shock ionisation timescales produced by the many coexisting stellar wind shocks; it is characterised by collisionally excited plasma temperatures of 1.81 ± 0.24 and 4.11 ± 1.98 keV and the 6.7 keV FeK α emission line. The outer region (Region #1) exhibits charge exchange emission (CXE) and is best described by a CXE+NEI model, with softer temperatures of 0.27 ± 0.04 and 1.09 ± 0.05 keV. Absorption column densities (N H) range between 1.84 and 2.24 ×10 22 cm −2 and the total (0.5-8.0 keV) diffuse luminosity of both regions is 9.6 (±0.3) × 10 33 erg s −1. Conclusions. Near the core, the hard emission likely originates from thermalised Wolf-Rayet (WR) stellar winds and intense windwind collisions in regions of high massive-star density, while the adiabatic expansion of the hot plasma and turbulent mixing with the denser, cooler ISM ends up softening it. The soft diffuse component extends farther out and may be associated with CXE processes, spatially correlated with regions of lower extinction. No hint of a non-thermal contribution is revealed. The analytical cluster wind model, using the observed WR population, predicts L CWM X 2.1 (±1.0) × 10 34 erg s −1 , a factor of ∼2.2 above the observed luminosity, consistent with the low-density, partially evacuated intracluster medium (ICM) inferred from the X-ray and infrared morphology. This low-density morphology at the cluster centre suggests that ultraviolet (UV) radiation can clear the ambient medium, producing an apparent outward displacement that likely reduces the L w-to-L X conversion efficiency, measured as η = L X /L w 4.3 × 10 −6 , approximately two orders of magnitude below that of the Cygnus OB2 association and systematically lower than every other massive star-forming region compiled here, resulting in comparatively subluminous diffuse X-ray emission.
