  The Cartwheel galaxy harbours more Ultra-Luminous X-ray sources (ULXs) than
any other galaxy observed so far, and as such it is a particularly interesting
target to study them. In this paper we analyse the three Chandra observations
of the brightest ULX (N10) in the Cartwheel galaxy, in light of current
theoretical models suggested to explain such still elusive objects. For each
model we derive the relevant spectral parameters. Based on self--consistency
arguments we can interpret N10 as an accreting binary system powered by a ~100
solar masses black hole. A young supernova strongly interacting with its
surroundings is a likely alternative, that can be discarded only with the
evidence of a flux increase from future observations.
