Re'Em Sari

Cosmic gamma ray bursts (GRBs) have fascinated scientists and the public alike since their discovery in the late 1960s. Their story is told here by some of the scientists who participated in their discovery and, after many decades of false starts, solved the problem of their origin. Fourteen chapters by active researchers in the field present a detailed history of the discovery, a comprehensive theoretical description of GRB central engine and emission models, a discussion of GRB host galaxies and a guide to how GRBs can be used as cosmological tools. Observations are grouped into three sets from the satellites CGRO, BeppoSAX and Swift, and followed by a discussion of multi-wavelength observations. This is the first edited volume on GRB astrophysics that presents a fully comprehensive review of the subject. Utilizing the latest research, Gamma-ray Bursts is an essential desktop companion for graduate students and researchers in astrophysics.

The detection of an afterglow, following a gamma-ray burst (GRB), can be described reasonably well by synchrotron emission from a relativistic spherical expanding blast wave, driven by an expanding fireball. We perform detailed calculations considering the emission from the whole region behind the shock front. We use the Blandford McKee self similar adiabatic solution to describe the fluid behind the shock. Using this detailed model, we derive expressions for the peak flux, and the peak frequency at a given observed time. These expressions provide important numerical corrections to previous, more simplified models. We calculate the observed light curve and spectra for several magnetic field models. We show that both the light curve and the spectra are flat near the peak. This rules out the interpretation of the optical peak of GRB970508 as the peak of the light curve, predicted by the existing fireball models. We calculate the observed image of a GRB afterglow. The observed image is bright near the edge and dimmer at the center, thus creating a ring. The contrast between the edge and the center is larger at high frequencies and the width of the ring is smaller.Comment: 32 page latex file including 14 figures and 2 table

The recently discovered GRB afterglow is believed to be described reasonably well by synchrotron emission from a slowing down relativistic shell that collides with an external medium. To compare theoretical models with afterglow observations we calculate here the broad band spectrum and corresponding light curve of synchrotron radiation from a power-law distribution of electrons in an expanding relativistic shock. Both the spectrum and the light curve consist of several power-law segments. The light curve is constructed under two limiting models for the hydrodynamical evolution of the shock: fully adiabatic and fully radiative. We compare the results with observations of $\gamma$-ray burst afterglows.Comment: 8 page latex file including 2 figure

We examine the relativistic decelerating shell model for γ-ray burst afterglow, and show how one can infer the model’s free parameters from afterglow observations.

We discuss the possibility that gamma-ray bursts result from internal shocks in ultra-relativistic matter. Using a simple model we calculate the temporal structure and we estimate the efficiency of this process. In this model the ultra-relativistic matter flow is represented by a succession of shells with random values of the Lorentz factor. We calculate the shocks that take place between those shells and we estimate the resulting emission. Internal shocks can produce the highly variable temporal structure observed in most of the bursts provided that the source emitting the relativistic flow is highly variable. The observed peaks are in almost one-to-one correlation to the activity of the emitting source. A large fraction of the kinetic energy is converted to radiation. The most efficient case is when an inner engine produces shells with comparable energy but very different Lorentz factors. It also gives the most preferable temporal structure.

Most current GRB models are based on the conversion, via shocks, of the energy of an ultra-relativistic flow to the observed radiation. We show that external shocks cannot produce the highly variable temporal structure observed in most GRBs. Internal shocks, on the other hand can produce this structure, provided that the inner engine that produce the relativistic flow generates an unsteady and irregular “wind.” However, internal shocks can convert only a fraction of the total energy to radiation. Most of the kinetic energy of the flow must be eventually dissipated via external shocks. We suggest that this is the origin of the observed afterglow. According to this Internal-external model the GRB and afterglow are not produced by the same process and hence they do not scale directly to each other. Furthermore, under certain conditions the initial phase of the afterglow might radiate in gamma rays and in these cases this signal will overlap as a smooth component with the variable signal produced by the internal shocks, leading to a combined scenario of the GRB itself.