The Cartwheel galaxy with XMM-Newton
  The extreme environment provided by the Cartwheel ring is analyzed to study
its X-ray and optical-UV properties. We compare the Cartwheel with the other
members of its group and study the system as a whole in the X-ray band. We
analyze the data of the Cartwheel galaxy obtained with XMM-Newton in two
different periods (December 2004 and May 2005). We focus on the X-ray
properties of the system and use the OM data to obtain additional information
in the optical and UV bands. We detect a total of 8 sources associated with the
Cartwheel galaxy and three in its vicinity, including G1 and G2, all at L >=
10^39 erg/s, that is the Ultra Luminous X-ray (ULX) source range. The brightest
ULX source has been already discussed elsewhere. The spectra of the next three
brightest ULX are well fitted by a power-law model with a mean photon index of
~2. We compare the XMM-Newton and Chandra datasets to study the long-term
variability of the sources. At least three sources vary in the 5 months between
the two XMM-Newton observations and at least four in the 4-year timeframe
between Chandra and XMM-Newton observations. One Chandra source disappears and
a new one is detected by XMM-Newton in the ring. Optical-UV colors of the
Cartwheel ring are consistent with a burst of star formation that is close to
reaching its maximum, yielding a mean stellar age of about 40 Myr. The inferred
variability and age suggest that high mass X-ray binaries are the counterparts
to the ULX sources. The 3 companion galaxies have luminosities in the range
10^39-40 erg/s consistent with expectations. The hot gas of the Cartwheel
galaxy is luminous and abundant (a few 10^8 Msol) and is found both in the
outer ring, and in the inner part of the galaxy, behind the shock wave front.
We also detect gas in the group with L_X ~10^40 erg/s.
