  The centrality dependence of spectra of identified particles in collisions
between ultrarelativistic heavy ions with a center of mass energy ($\sqrt{s}$)
of 39 and 11.5 $AGeV$ is analyzed in the core - corona model. We show that at
these energies the spectra can be well understood assuming that they are
composed of two components whose relative fraction depends on the centrality of
the interaction: The core component which describes an equilibrated quark gluon
plasma and the corona component which is caused by nucleons close to the
surface of the interaction zone which scatter only once and which is identical
to that observed in proton-proton collisions. The success of this approach at
39 and 11.5 $AGeV$ shows that the physics does not change between this energy
and $\sqrt{s}=200~ AGeV$ for which this model has been developed (Aichelin
2008). This presents circumstantial evidence that a quark gluon plasma is also
created at center of mass energies as low as 11.5 $AGeV$.
