Relation between E_p, z and L_iso fit with the X-ray data of the bursts in our sample, represented by colored circles. We assumed a sBPL spectral model. The straight green line represents the best-fit line from the linear fit, while the green-shaded area shows the 3σ_sc scatter region of the relation. The relation has been extrapolated to higher energies, where GBM short GRB data are represented with green squares.