Hard X-Rays from Ultracompact H II Regions in W49A

We report the Chandra detection of hard X-ray emission from the Welch ring in W49A, an organized structure of ultracompact (UC) H II regions containing a dozen nascent early-type stars. Two UC H II regions are associated with hard X-ray emission in a deep Advanced CCD Imaging Spectrometer (ACIS) image exposed for ~96.7 ks. One of the two X-ray sources has no near-infrared counterpart and is extended by ~5'', or ~0.3 pc, at a distance of ~11.4 kpc, which is spatially aligned with the cometary radio continuum emission associated with the UC H II region. The X-ray spectrum of the emission, when fit with a thermal model, indicates a heavily absorbed plasma with extinction of ~5 × 1023 cm-2, temperature ~7 keV, and X-ray luminosity in the 3.0-8.0 keV band of ~3×1033 ergs s-1. Both the luminosity and the size of the emission resemble the extended hard emission found in UC H II regions in Sagittarius B2, yet they are smaller by an order of magnitude than the emission found in massive star clusters such as NGC 3603. Three possibilities are discussed for the cause of the hard extended emission in the Welch ring: an ensemble of unresolved point sources, shocked interacting winds of the young O stars, and a wind-blown bubble interacting with ambient cold matter.