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We present the diffuse X-ray emission identified in Chandra observations of the young, massive Galactic star cluster Westerlund 1. After removing point-like X-ray sources down to a completeness limit of 2e31 erg/s, we identify 3e34 erg/s (2--8 keV) of diffuse emission. The spatial distribution of the emission can be described as a slightly-elliptical Lorentzian core with a half-width half-maximum along the major axis of 25+/-1", similar to the distribution of point sources in the cluster, plus a 5' halo of extended emission. The spectrum of the diffuse emission is dominated by a hard continuum component that can be described as a kT>3 keV thermal plasma that has a low iron abundance (<0.3 solar), or as non-thermal emission that could be stellar light that is inverse-Compton scattered by MeV electrons. Only 5% of the flux is produced by a kT=0.7 keV plasma. The low luminosity of the thermal emission and the lack of a 6.7 keV iron line suggests that <40,000 unresolved stars with masses between 0.3 and 2 Msun are present in the cluster. Moreover, the flux in the diffuse emission is a factor of two lower than would be expected from a supersonically-expanding cluster wind, and there is no evidence for thermal remnants produced by supernovae. Less than 1e-5 of the mechanical luminosity of the cluster is dissipated as 2--8 keV X-rays, leaving a large amount of energy that either is radiated at other wavelengths, is dissipated beyond the bounds of our image, or escapes into the intergalactic medium.

Diffuse, Nonthermal X‐Ray Emission from the Galactic Star Cluster Westerlund 1