We present a systematic study on magnetic fields in Gamma-Ray Burst (GRB) external forward shocks (FSs). There are 60 (35) GRBs in our X-ray (optical) sample, mostly from Swift. We use two methods to study ǫ B (fraction of energy in magnetic field in the FS). 1. For the X-ray sample, we use the constraint that the observed flux at the end of the steep decline is ≥ X-ray FS flux. 2. For the optical sample, we use the condition that the observed flux arises from the FS (optical sample light curves decline as ∼ t −1 , as expected for the FS). Making a reasonable assumption on E (jet isotropic equivalent kinetic energy), we converted these conditions into an upper limit (measurement) on ǫ B n 2/(p+1) for our X-ray (optical) sample, where n is the circumburst density and p is the electron index. Taking n = 1 cm −3 , the distribution of ǫ B measurements (upper limits) for our optical (X-ray) sample has a range of ∼ 10 −8 − 10 −3 (∼ 10 −6 − 10 −3 ) and median of ∼ few × 10 −5 (∼ few × 10 −5 ). To characterize how much amplification is needed, beyond shock compression of a seed magnetic field ∼ 10µG, we expressed our results in terms of an amplification factor, AF , which is very weakly dependent on n (AF ∝ n 0.21 ). The range of AF measurements (upper limits) for our optical (X-ray) sample is ∼ 1 − 1000 (∼ 10 − 300) with a median of ∼ 50 (∼ 50). These results suggest that some amplification, in addition to shock compression, is needed to explain the afterglow observations.

We present a Monte Carlo (MC) code we wrote to simulate the photospheric process and to study the photospheric spectrum above the peak energy. Our simulations were performed with a photon to electron ratio N γ /N e = 10 5 , as determined by observations of the GRB prompt emission. We searched an exhaustive parameter space to determine if the photospheric process can match the observed high-energy spectrum of the prompt emission. If we do not consider electron re-heating, we determined that the best conditions to produce the observed high-energy spectrum are low photon temperatures and high optical depths. However, for these simulations, the spectrum peaks at an energy below 300 keV by a factor ∼ 10. For the cases we consider with higher photon temperatures and lower optical depths, we demonstrate that additional energy in the electrons is required to produce a power-law spectrum above the peak-energy. By considering electron re-heating near the photosphere, the spectrum for these simulations have a peak-energy ∼ 300 keV and a power-law spectrum extending to at least 10 MeV with a spectral index consistent with the prompt emission observations. We also performed simulations for different values of N γ /N e and determined that the simulation results are very sensitive to N γ /N e . Lastly, in addition to Comptonizing a Blackbody spectrum, we also simulate the Comptonization of a f ν ∝ ν −1/2 fast cooled synchrotron spectrum. The spectrum for these simulations peaks at ∼ 10 4 keV, with a flat spectrum f ν ∝ ν 0 below the peak energy.

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We study the spectra of photospheric emission from highly relativistic gamma-ray burst outflows using a Monte Carlo (MC) code. We consider the Comptonization of photons with a fast cooled synchrotron spectrum in a relativistic jet with realistic photon to electron number ratio N γ /N e = 10 5 , using mono-energetic protons which interact with thermalised electrons through Coulomb interaction. The photons, electrons and protons are cooled adiabatically as the jet expands outwards. We find that the initial energy distribution of the protons and electrons do not have any appreciable effect on the photon peak energy E γ,peak and the power-law spectrum above E γ,peak . The Coulomb interaction between the electrons and the protons does not affect the output photon spectrum significantly as the energy of the electrons is elevated only marginally. E γ,peak and the spectral indices for the low and high energy power-law tails of the photon spectrum remain practically unchanged even with electron-proton coupling. Increasing the initial optical depth τ in results in slightly shallower photon spectrum below E γ,peak and fewer photons at the high-energy tail, although f ν ∝ ν −0.5 above E γ,peak and up to ∼ 1 MeV, independent of τ in . We find that E γ,peak determines the peak energy and the shape of the output photon spectrum. Lastly, we find that our simulation results are quite sensitive to N γ /N e , for N e = 3 × 10 3 . For almost all our simulations, we obtain an output photon spectrum with power-law tail above E γ,peak extending up to ∼ 1 MeV.

Within the first 10 days after Swift discovered the jetted tidal disruption event (TDE) Sw J1644+57, simultaneous observations in the radio, near-infrared, optical, X-ray and gammaray bands were carried out. These multiwavelength data provide a unique opportunity to constrain the emission mechanism and make-up of a relativistic super-Eddington jet. We consider an exhaustive variety of radiation mechanisms for the generation of X-rays in this TDE, and rule out many processes such as SSC, photospheric and proton synchrotron. The infrared to gamma-ray data for Sw J1644+57 are consistent with synchrotron and external-inverse-Compton (EIC) processes provided that electrons in the jet are continuously accelerated on a time scale shorter than ∼ 1% of the dynamical time to maintain a power-law distribution. The requirement of continuous electron acceleration points to magnetic reconnection in a Poynting flux dominated jet. The EIC process may require fine tuning to explain the observed temporal decay of the X-ray lightcurve, whereas the synchrotron process in a magnetic jet needs no fine tuning for this TDE.