X-ray lines, progenitor and beaming of Gamma-Ray Bursts

Abstract

X-ray emission features have been detected in at least five GRB afterglows. Their detection and properties are of great importance in our understanding of the nature of the GRB progenitor. Moreover, they provide us with an un-collimated source of photons, allowing us to put firm constraints on the total energy produced in GRBs, and therefore on their degree of collimation. I will review the radiation mechanisms and geometries proposed as sources of the line emission, and discuss the energy budget of the lines and their implication for the overall energetics of the phenomenon.