Dr. Minhajur Rahaman

Gamma ray bursts are the most energetic electromagnetic phenomenon in the Universe. A major puzzle since their discovery five decades back is the origin of prompt-emission. One of the leading candidates for energy dissipation that powers the prompt-phase is internal shocks, in which the central source produces an ultra-relativistic outflow with varying Lorentz factors, leading to collisions at large distances from the source. Each collision produces a pair of shock fronts viz., a reverse and forward shock front with different strengths, leading to different physical conditions in the two shocked regions. In this talk I will present hydrodynamic treatment of internal shocks, which naturally reproduces the most prominent observed features of prompt GRB emission. We show that optically-thin synchrotron emission from both shocked regions can explain the pulse shapes (the fast rise, slow decay, and large diversity between different pulses), time-evolution of the ¿F¿ peak flux and photon-energy, and the time-integrated spectrum. Particularly, two features commonly observed in GRB spectra can also be accounted for : (i) a sub-dominant low-energy spectral component (often interpreted as “photospheric”-like), or (ii) a doubly-broken power-law spectrum with the low-energy spectral slope approaching the slow cooling limit.