Patrick

Superbubbles are powered by the stellar winds and subsequent supernovae of a massive stellar population and are often argued as strong candidates for Galactic cosmic-ray acceleration sites. The recent detection of TeV $gamma$-rays from 30~Dor~C in the Large Magellanic Cloud by the {it High Energy Stereoscopic System} ({it H.E.S.S.}) has shown that superbubbles can and do accelerate particles up to very high cosmic-ray energies, and are a new and important source class in TeV astronomy. However, the dominant production mechanism (i.e., hadronic or leptonic) is still unclear. The answer to this question is locked in the unique synchrotron X-ray shell of 30~Dor~C. The widths of the synchrotron emission regions are directly related to the magnetic field, which is a crucial parameter assessing dominant $gamma$-ray emission mechanism. In this talk we will present a study of the synchrotron emission region widths in 30~Dor~C using several hundred ks of archival X-ray data from {it XMM-Newton}. We constructed radial emission profiles from various regions of the synchrotron shell, fitted emission models to determine the widths, and derived $B$-field values in the downstream regions using appropriate models. The resulting low $B$-field estimates, of the order of a few $mu$G, favour a leptonic origin for the $gamma$-ray emission. Hadronic cosmic rays are likely to be accelerated as well, but the low density inside the bubble suppresses their emissivity.