Thermal and nonthermal X-rays from the Large Magellanic Cloud superbubble 30 Doradus C

We report on the discovery of thermal and nonthermal X-rays from the shells of the superbubble (SB) 30 Dor C in the LMC. The X-ray morphology is a nearly circular shell with a radius of similar to40 pc, which is bright on the northern and western sides. The spectra of the shells differ from region to region. The southern shell shows clear emission lines and is well fitted with a model of a thin thermal plasma (kT=0.21 keV) in nonequilibrium ionization plus a power-law component. This thermal plasma is located inside of the Halpha emission, which is the outer edge of the shell of the SB. The northern and western sides of the SB are dim in Halpha emission but are bright in nonthermal (power-law) X-rays with a photon index of 2.1-2.9. The nonthermal X-ray shell traces the outer boundary of the radio shell. These features of thin thermal and nonthermal X-rays are similar to those of SN 1006, a prototype of a synchrotron X-ray shell, but the nonthermal component of 3 0 Dor C is about 10 times brighter than that of SN 1006. 30 Dor C is the first candidate of an extragalactic SB, in which energetic electrons are accelerating in the shell. The age is much older than that of SN 1006, and hence the particle acceleration time in this SB may be longer than those in normal shell-like supernova remnants. We found pointlike sources associated with some of the tight star clusters. The X-ray luminosity and spectrum are consistent with those of young clusters of massive stars. Pointlike sources with nonthermal spectra are also found in the SB. These may be background objects (active galactic nuclei) or stellar remnants (neutron stars or black holes).