Diffuse, nonthermal X-ray emission from the galactic star cluster westerlund 1

We present the diffuse X-ray emission identified in Chandra observations of the young, massive Galactic star cluster Westerlund 1. After removing pointlike X-ray sources down to a completeness limit of ≈2 × 10³¹ ergs s^-1, we identify (3 ± 1) × 10³⁴ ergs s^-1 (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 at 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 nonthermal 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 suggest that ≲40,000 unresolved stars with masses between 0.3 and 2 M_⊙ are present in the cluster, fewer than previously estimated. Moreover, the flux in the diffuse emission is a factor of several lower than would be expected from a supersonically expanding cluster wind, and there is no evidence for thermal remnants produced by supernovae. Less than 10^-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.