  We used a combination of high-resolution optical images acquired with the
Hubble Space Telescope and near-IR wide-field data to investigate the stellar
density profile and the population of blue straggler star (BSS) in the Galactic
globular cluster NGC6256, with the aim of probing its current stage of internal
dynamical evolution. We found that the inner stellar density profile
significantly deviates from a King model while is well reproduced by a steep
cusp with a power-law slope alpha=-0.89, thus implying that the cluster is
currently in the post core-collapse (PCC) phase. This is also confirmed by the
very high segregation level of the BSS population measured through the A+
parameter. We also found that the distribution of BSSs in the color-magnitude
diagram is characterized by a collimated blue sequence and a red more sparse
component, as already observed in other three PCC clusters. The comparison with
appropriate collisional models demonstrates that the vast majority of the BSSs
lying along the collimated blue sequence is consistent with a generation of
coeval (1 Gyr-old) stars with different masses originated by an event that
highly enhanced the collisional rate of the system (i.e. the core collapse).
This study confirms that the segregation level of BSSs is a powerful dynamical
diagnostic also of star cluster in a very advanced stage of dynamical
evolution. Moreover, it pushes forward the possibility of using the morphology
of the BSS in the color-magnitude diagram as a tracer of the core-collapse and
subsequent dynamical evolutionary phases.
