  Ring galaxies are fascinating laboratories: a catastrophic impact between two
galaxies (one not much smaller than the other) has produced fireworks
especially in the larger one, when hit roughly perpendicularly to the plane. We
analyze the point sources, produced by the starburst episode following the
impact, in the rings of seven galaxies and determine their X-ray luminosity
function (XLF). In total we detect 63 sources, of which 50 have luminosity L$_X
\geq 10^{39}$ erg s$^{-1}$, classifying them as ultra luminous X-ray sources
(ULXs). We find that the total XLF is not significantly different from XLFs
derived for other kinds of galaxies, with a tendency of having a larger
fraction of high X-ray luminosity objects. Both the total number of ULXs and
the number of ULXs per unit star formation rate are found in the upper envelope
of the more normal galaxies distribution. Further analysis would be needed to
address the issue of the nature of the compact component in the binary system.

High Energy Astrophysical Phenomena;Astrophysics of Galaxies