Models

Since their discovery, there has been considerable speculation concerning the source of gamma ray bursts. It was difficult to test those speculations when we did not even know how far away they were. Now that more quantitative data are becoming available, we can begin to sharpen the discussion.

Relativistic Shock Fireball Model
There is growing agreement that the transients observed following gamma ray bursts at various other wavelengths can be accounted for by a relativistic fireball model, as illustrated in the adjacent figure. In this model some central source or "engine" deposits a very large amount of energy in a small volume of space. That produces a "fireball" expanding at relativistic velocities and this fireball is responsible for the observed transients. The mechanism thought to produce the fireball is illustrated schematically in the following figure and described in more detail in this animation (Erin fireball).

Internal Shocks and the Gamma Ray Burst
In this so-called shock fireball model, a central engine not yet specified produces the energy for the burst in a very compact region (we shall discuss possible central engines below). The central engine emits jets moving at relativistic velocity. Shockwaves are produced as shells of ejected matter overtake and collide with each other in the jet. These are called internal shocks because they are thought to occur in the immediate vicinity of the central engine. It is these internal shocks that are responsible for producing the gamma rays of the burst. One plausible mechanism is that photons are produced by synchrotron processes as charged particles are accelerated in magnetic fields of the jet. These photons are then boosted to higher (gamma-ray) energies and focused in the forward direction by relativistic effects such as those that we discussed in conjunction with jets from the crab pulsar in Chapter 14 and from AGNs in Chapter 17. The result is an intense, focused pulse of gamma rays.

Why Relativistic?

In the fireball model described in the adjacent paragraphs, the jet that produces the fireball is thought to move at relativistic velocity. If the jet were not relativistic, the gamma rays produced would interact either with each other or with the surrounding matter to produce many lower-energy photons, leaving no gamma rays. Such a process is called downscattering because the effect of the scattering is to produce more photons of lower average energy. Such downscattering must be avoided if the model is to account for the burst of gamma rays seen by distant observers.

External Shocks and Afterglows
The burst of gamma rays produced in this manner corresponds to the bursts detected by gamma ray detectors. As the relativistic jet continues onward, it passes through the interstellar medium at velocities greater than the speed of sound there, so external shocks are produced as the jet plows through the interstellar medium. It is these external shocks (so called because they occur in the surrounding interstellar medium, external to the engine producing the actual gamma ray burst) that are responsible for producing the afterglows at longer wavelengths.
What Drives the Fireball?
The fireball model accounts for the behavior of the transients and perhaps the gamma ray burst itself, but does not directly specify the energy source that drives the fireball. That is, the fireball model leaves open the nature of the central engine that powers the burst, except that it requires the central engine to be able to produce the relativistic jets and shocks assumed by the model. Several specific types of "engines" have been proposed as the source of the initial fireball energy. The pressing question now is to determine which central engine is most consistent with the observations.