Prompt emission of GRBs

Gamma-ray bursts (GRBs) were discovered in late 1960's as flashes of gamma-rays and hard Xrays, known as the prompt phase. Later they were observed throughout the electromagentic spectrum as afterglows. The precise localization of GRBs proved  their cosmic origin, which together with high observed flux, placed GRBs as the most energetic events in the universe.  More than a dozen satellites have been flown (e.g., Swift, Fermi), numerous afterglow and host galaxy studies  have been carried out to identify the physical mechanism of GRBs. These have given a phenomenological perspective regarding the origin of GRBs. From these extensive studies we know that GRBs have two classes.  Type I GRBs are formed by merging of binary compact objects, while Type II GRBs are formed due to collapse of very  massive Wolf-Rayet (WR) stars. In spite of these discoveries the central engine and the emission mechanism of  the prompt phase of GRBs remain a puzzle. These are addressed from a data analyst's point of view, using the set of  Type II GRBs, which have certain advantages over Type I --- (i) larger set, (ii) longer prompt phase, (iii) brighter  afterglow phase.  I shall argue that meaningful physical pictures can be identified by closely examining the temporal structures and studying the spectral evolution, rather than analyzing the average spectrum. I shall show that some reasonable assumptions of the spectral evolution can lead to correct identification of the underlying radiation mechanism. This formalism has led to some very recent and exciting results like improvement of the GRB luminosity indicator,  explanation of very high energy (GeV) emission.