We describe a general method for modelling γ -ray burst (GRB) prompt emission, and determine the range of magnetic field strength, electron energy, Lorentz factor of the source and the distance of the source from the central explosion that is needed to account for the prompt γ -ray emission of a typical long-duration burst. We find that for the burst to be produced via the synchrotron process unphysical conditions are required -the distance of the source from the centre of the explosion (R γ ) must be larger than ∼10 17 cm and the source Lorentz factor 10 3 ; for such a high Lorentz factor the deceleration radius (R d ) is less than R γ even if the number density of particles in the surrounding medium is as small as ∼0.1 cm −3 . The result, R γ > R d , is in contradiction with the early X-ray and optical afterglow data that show that γ -rays precede the afterglow flux that is produced by a decelerating forward shock. This problem for the synchrotron process applies to all long GRBs other than those that have the low-energy spectrum precisely ν −1/2 . In order for the synchrotron process to be a viable mechanism for long bursts, the energy of electrons radiating in the γ -ray band needs to be continuously replenished by some acceleration mechanism during much of the observed spike in GRB light curve -this is not possible if GRB-prompt radiation is produced in shocks (at least the kind that has been usually considered for GRBs) where particles are accelerated at the shock front and not as they travel downstream and emit γ -rays, but might work in some different scenarios such as magnetic outflows.The synchrotron-self-Compton (SSC) process fares much better. There is a large solution space for a typical GRB-prompt emission to be produced via the SSC process. The prompt optical emission accompanying the burst is found to be very bright ( 14 mag; for z ∼ 2) in the SSC model, which exceeds the observed flux (or upper limit) for most GRBs. The prompt optical is predicted to be even brighter for the subclass of bursts that have the spectrum f ν ∝ν α with α ∼ 1 below the peak of νf ν . Surprisingly, there are no SSC solutions for bursts that have α ∼ 1/3; these bursts might require continuous or repeated acceleration of electrons or some physics beyond the simplified, although generic, SSC model considered in this work. Continuous acceleration of electrons can also significantly reduce the optical flux that would otherwise accompany γ -rays in the SSC model.

We calculate the reverse shock (RS) synchrotron emission in the optical and the radio wavelength bands from electron–positron pair‐enriched gamma‐ray burst ejecta with the goal of determining the pair content of gamma‐ray bursts (GRBs) using early‐time observations. We take into account an extensive number of physical effects that influence radiation from the RS‐heated GRB ejecta. We find that optical/infrared flux depends very weakly on the number of pairs in the ejecta, and there is no unique signature of ejecta pair enrichment if observations are confined to a single wavelength band. It may be possible to determine if the number of pairs per proton in the ejecta is ≳100 by using observations in optical and radio bands; the ratio of flux in the optical and radio at the peak of each respective RS light curve is dependent on the number of pairs per proton. We also find that over a large parameter space, RS emission is expected to be very weak; GRB 990123 seems to have been an exceptional burst in that only a very small fraction of the parameter space produces optical flashes this bright. Also, it is often the case that the optical flux from the forward shock is brighter than the RS flux at deceleration. This could be another possible reason for the paucity of prompt optical flashes with a rapidly declining light curve at early times as was seen in GRBs 990123 and 021211. Some of these results are a generalization of similar results reported in Nakar & Piran.

The Swift satellite has enabled us to follow the evolution of gamma-ray burst (GRB) fireballs from the prompt gamma-ray emission to the afterglow phase. The early x-ray and optical data obtained by telescopes aboard the Swift satellite show that the source for prompt gamma-ray emission, the emission that heralds these bursts, is short lived and that its source is distinct from that of the ensuing, long-lived afterglow. Using these data, we determine the distance of the gamma-ray source from the center of the explosion. We find this distance to be 1e15-1e16 cm for most bursts and we show that this is within a factor of ten of the radius of the shock-heated circumstellar medium (CSM) producing the x-ray photons. Furthermore, using the early gamma-ray, x-ray and optical data, we show that the prompt gamma-ray emission cannot be produced in internal shocks, nor can it be produced in the external shock; in a more general sense gamma-ray generation mechanisms based on shock physics have problems explaining the GRB data for the ten Swift bursts analyzed in this work. A magnetic field dominated outflow model for GRBs has some attractive features, although the evidence in its favor is inconclusive. Finally, the x-ray and optical data allow us to provide an upper limit on the density of the CSM of about 10 protons per cubic cm at a distance of about 5e16 cm from the center of explosion.Comment: Accepted to MNRAS Letters. 6 pages, 2 figures, & 2 table

We describe our attempt to determine if γ -ray burst (GRB) and afterglow emissions could both arise in external shocks for simple GRBs -bursts consisting of just a few peaks in their light curves. We calculate peak flux and peak frequency during the γ -ray burst for 10 well-observed bursts using the same set of parameters that are determined from modelling afterglow emissions. We find the γ -ray emission properties for 970508 (which had a singlepeak light curve) fit nicely with the extrapolation of its afterglow data, and therefore this burst was probably produced in the external shock. One can explain two other bursts in this sample as forward shock synchrotron emission provided that the magnetic field parameter during the burst is close to equipartition, and larger by a factor ∼10 2 than the afterglow value at ∼1 d. The remaining seven bursts cannot be explained in the external shock model even if we allow the energy fraction in electrons and magnetic field and the density of the surrounding medium to take on any physically permitted value; the peak of the spectrum is above the cooling frequency, therefore the peak flux is independent of the latter two of these parameters, and is smaller by about an order of magnitude than the observed values. We have also considered inverse-Compton scattering in forward and reverse shock regions and find that it can explain the γ -ray emission for a few bursts, but requires the density to be 1-2 orders of magnitude larger than a typical Wolf-Rayet star wind and much larger than permitted by late afterglow observations.We have also calculated emission from the reverse shock for these ten bursts and find the flux in the optical band for more than half of these bursts to be between 9th and 12th magnitude at the deceleration time if the reverse shock microphysics parameters are the same as those found from afterglow modelling and the deceleration time is of the order of the burst duration. However, the cooling frequency in the reverse shock for most of these bursts is below the optical band, and therefore the observed flux decays rapidly with time (as ∼t −3 ) and is unobservable after a few deceleration times. It is also possible that the deceleration time is much larger than the burst duration, in which case we expect weak reverse shock emission.

Data from the Swift satellite have enabled us for the first time to provide a complete picture of the gamma‐ray (γ‐ray) burst emission mechanism and its relationship with the early afterglow emissions. We show that γ‐ray photons for two bursts, 050126 and 050219A, for which we have carried out detailed analysis were produced as a result of the synchrotron self‐Compton process in the material ejected in the explosion when it was heated to a mildly relativistic temperature at a distance from the centre of explosion of order the deceleration radius. Both of these bursts exhibit rapidly declining early X‐ray afterglow light curves; this emission is from the same source that produced the γ‐ray burst. The technique that we exploit to determine this is very general and makes no assumption about any particular model for γ‐ray generation except that the basic radiation mechanism is some combination of synchrotron and inverse Compton processes in a relativistic outflow. For GRB 050219A we can rule out the possibility that energy from the explosion is carried outward by magnetic fields, and that the dissipation of this field produced the γ‐ray burst.