Magnetic fireball: The acceleration efficiency of hydromagnetic outflows in GRB sources

Gamma-ray bursts (GRBs) have been inferred to arise in highly collimated, ultrarelativistic jets that emanate from the vicinity of a solar-mass compact object. Electromagnetic stresses are the most plausible candidate for extracting rotational energy at the source and converting it into outflow kinetic energy. Two questions that need to be answered in order for this process to be well understood are: what determines the terminal Lorentz factor of the flow? What is the asymptotic value of the Poynting-to-matter energy flux ratio? We discuss the general characteristics of the relativistic magnetohydrodynamic (MHD) solutions that, together with previously obtained exact results, help to shed light on these questions