GRB: A luminous candle? Recent experimental evidences

Gamma-Ray Burst (GRB) emit intense flashes of γ-rays typically in the 10 keV -10 MeV energy range and lasting from fraction of seconds to a few hundred seconds. Their prompt γ-ray emission is followed far much longer timescales by considerable emission at lower frequencies (X-ray, optical, IR and radio): the GRB afterglow. The detection of GRB optical counterparts has allowed to measure (since 1997) their redshifts, definitely showing that they are cosmological sources. The broad redshift interval (presently extending out to z ∼ 4.5) and the huge power emitted in the prompt γ-ray phase, make these sources excellent candidates to probe the distant universe. The continuous progress on the determination of the main GRB phenomenological (temporal and spectral) properties is providing new clues on their nature: in particular intrinsic correlations have been recently revealed between the GRB spectral characteristics and their global energetics, which can potentially allow to use GRB as rulers to measure the cosmological parameters. In this presentation a brief review of these correlations and of their cosmological significance is given. It is stressed the relevance