This project will model the emission from Gamma-Ray Burst (GRB) sources at various epochs and in different energy bands. It makes a strong effort to develop analytical, semi-analytical and numerical frameworks. The work has three main thrusts: 1) to develop a comprehensive model of GRB afterglows by calculating characteristic break frequencies, flux power-law indices and the bolometric flux evolution in the jitter emission paradigm; 2) to understand the non-Fermi acceleration of electrons in collisionless GRB shocks; and 3) to develop a semi-analytical and numerical framework for studies of GRB transient emission, covering the prompt and X-ray flare phases. Accurate modeling of temporal, spectral and polarization evolution will explore whether the models are consistent with the observed correlations. The results will be valuable for improved physical understanding and interpretation of observational results, and ultimately for uncovering the mystery of the origin of GRBs.
Graduate student training, education and research is the dominant component of this study, which will support three students for three inter-related sub-projects. During the study, the team will develop a display at their department, along with appropriate web resources, to describe the GRB phenomenon in a non-traditional way using every-day physics. Additional impacts will come from the development of theoretical tools that will influence many areas of plasma astrophysics.
Gamma-Ray Bursts (GRBs) are the most powerful and enigmatic phenomenon in the universe. They are likely produced in explosions of massive stars and are so bright that they are seen across the entire universe. Their "central engines" are hidden from us by a surrounding, enormously opaque fireball consisting of baryons, electron-positron pairs and photons, which expands into the ambient medium with relativistic (i.e, very close to the speed of light) velocities. Later, when this fireball expands tremendously, the gas inside it becomes dilute. At this moment, the fireball becomes transparent and we detect a bunch of high-energy photons -- the prompt GRB. The prompt phase is very short (durations range from tens of milliseconds to hundreds of seconds) and exhibits very rapid erratic variations of brightness and photon energy. As the fireball keeps expanding, it interacts with an external medium to produce a delayed afterglow, which can last from days to years after the main event. This NSF-sponsored project was devoted to the studies of the prompt phase and the early afterglow. Despite extensive studies over the past decade, we still do not know for sure how the observed radiation is produced and what causes the rapid variability of the prompt GRB spectra and intensity. In the most widely assumed model, the prompt GRBs are produced inside the expanded fireball in so-called "internal shock waves" -- sharp discontinuities of the gas parameters, such as the velocity, density and temperature. When such shock waves propagate through a medium, they do two crucial things: (i) produce super-strong magnetic fields and (ii) accelerate particles (electrons, protons) to very high energies. The energetic electrons moving in magnetic fields radiate photons we ultimately detect as a GRB. There has been a number of outstanding questions in the theory of GRBs: how do the collisionless relativistic shocks form in the first place, how do they generate magnetic fields from scratch, how are the particles accelerated, what is there effect on the pre-shock medium and, of course, what is the mechanism of radiation production? In the course of the long-term investigation, it has been realized the the so-called Weibel instability plays a key role in shaping the structure of the shocks, their evolution and conditions for efficient particle acceleration. Further it has been realized the the conventional theory of synchrotron emission is invalid at such shocks on physics grounds, and it is also at odds with observational data. Based on our model of a Weibel shock, we have also proposed a new radiation mechanism called "jitter radiation" which can naturally account for the observed non-synchrotron spectra. In subsequent studies we developed a comprehensive numerical model of GRBs which takes into account relativistic kinematics of GRB outflows, the Weibel shock theory and jitter radiation mechanism. At present, this model is the only one which can simultaneously explain the GRB spectra and some special correlations of certain spectral parameters deduced from time-resolved analysis of GRB observational data. In our study we also went further in a variety of new directions which lead us to much deeper and broader understanding of GRBs. Overall, over twenty refereed papers were published during the investigation, but we want to highlight just one here. The Weibel instability can be studied in lab experiments at modern Petawatt-scale laser facilities such as the National Ignition Facility and a few others. These lasers can produce relativistic electron beams with Lorentz factors of about a hundred, similar to those in GRBs, and the Weibel turbulence has already been observed in some laser-plasma experiments. Amazingly, the laser-plasma parameters are very similar to those in GRB shocks, setting a characteristic length-scale to be about ten microns in laser plasmas and a few millimeters in GRBs. Apparently, not much scaling is needed between astrophysical phenomena and lab experiments. Producing a Weibel-mediated collisionless shock is clearly a feasible experiment with a typical centimeter-size target. Lighting up a GRB in a lab -- what can be more fascinating than this!
