Accepted for publication in Reviews of Modern Physics
astro-ph/0405503

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Abstract. Gamma-Ray Bursts (GRBs), short and intense pulses of low energy -rays, have fascinated astronomers and astrophysicists since their unexpected discovery in the late sixties. During the last decade, several space missions: BATSE (Burst and Transient Source Experiment) on Compton Gamma-Ray Observatory, BeppoSAX and now HETE II (High-Energy Transient Explorer), together with ground optical, infrared and radio observatories have revolutionized our understanding of GRBs showing that they are cosmological, that they are accompanied by long lasting afterglows and that they are associated with core collapse Supernovae. At the same time a theoretical understanding has emerged in the form of the fireball internal-external shocks model. According to this model GRBs are produced when the kinetic energy of an ultra-relativistic flow is dissipated in internal collisions. The afterglow arises when the flow is slowed down by shocks with the surrounding circum-burst matter. This model has numerous successful predictions like the prediction of the afterglow itself, the prediction of jet breaks in the afterglow light curve and of an optical flash that accompanies the GRBs themselves. In this review I focus on theoretical aspects and on physical processes believed to take place in GRBs.

Table of Contents

• INTRODUCTION
• OBSERVATIONS
• Prompt Emission
• The Afterglow
• Hosts and Distribution
• Energetics
• THE GLOBAL PICTURE - GENERALLY ACCEPTED INGREDIENTS
• RELATIVISTIC EFFECTS
• Compactness and relativistic motion
• Relativistic time effects
• Relativistic Beaming and the Patchy Shell Model
• PHYSICAL PROCESSES
• Relativistic Shocks
• Particle Acceleration
• Synchrotron
• Inverse Compton
• Quasi-Thermal Comptonization
• Polarization from Relativistically Moving Sources
• THE GRB AND THE PROMPT EMISSION
• Internal vs. External Shocks
• Internal Shocks
• External Shocks
• The Transition from Internal Shocks to External Shocks
• Prompt Polarization
• THE AFTERGLOW
• Relativistic Blast Waves and the Blandford-McKee solution
• Light Curves for the "Standard" Adiabatic Synchrotron Model
• Light Curves for the early radiative phase
• Light Curve During the Newtonian transition
• Generalizations: I. Winds
• Generalizations: II. Energy injection and refreshed shocks
• Generalizations: III. Inhomogeneous density profiles
• Generalizations: IV. Jets
• Generalizations: V. Angular Dependent Jets and the Structured Jet Model
• Afterglow Polarization - a tool that distinguished between the different jet models
• Orphan Afterglows
• Generalizations: VI. Additional Physical Processes
• ADDITIONAL EMISSION FROM GRBs
• TeV -rays
• Neutrinos
• Cosmic Rays and Ultra High Energy Cosmic Rays
• Gravitational Radiation
• MODELS OF INNER ENGINES
• Black hole accretion
• The Pulsar Model
• Rotating black holes and the Blandford Znajek mechanism
• The Collapsar Model
• The Supranova Model
• Merging neutron stars
• OPEN QUESTIONS AND FUTURE PROSPECTS
• REFERENCES