A thorough study of the intriguing X-ray emission from the Cartwheel   ring
  We present the results from the high resolution Chandra observation of the Cartwheel galaxy. Many individual sources are resolved in the image, mostly associated with the outer ring. All detected sources have a very high X-ray luminosity ($\geq 10^{39}$ erg s$^{-1}$)that classifies them as Ultra Luminous X-ray sources (ULX). The brightest of them is possibly the most luminous individual non-nuclear source observed so far, with L$_X \sim 10^{41}$ erg s$^{-1}$ (at D=122 Mpc). The spatial extent of this source is consistent with a point source at the Chandra resolution. The luminosity function of individual X-ray sources extends about an order of magnitude higher than previously reported in other galaxies. We discuss this in the context of the "universal" luminosity function for High Mass X-ray Binaries and we derive a Star Formation Rate higher than in other starburst galaxies studied so far. A diffuse component, associated with hot gas, is present. However, deeper observations that we will obtain with XMM-Newton are needed to constrain its properties. 
Anna Wolter, Ginevra Trinchieri (INAF-Osservatorio Astronomico di   Brera, Milano)