Gamma Ray Bursts - A Primer

One of the deep and abiding mysteries that nature has to offer is the existence of Gamma Ray Bursts (GRBs). They were first detected over twenty years (Klebesadel, Strong, and Olson 1973), and have since attracted the interest of both observers and theorists. To date, more than 2000 papers have been published about them, yet no one knows for sure what GRB's are, where they come from, or even whether or not they are a single class of phenomena. Indeed, the characteristic distance to GRB sources , and hence the energy required to produce the bursts, remains indeterminate to many orders of magnitude. The persistence of the GRB mystery reflects the difficulty of observing brief transients that recur either infrequently or not at all.

Before the launch of the Compton Gamma-Ray observatory (CGRO), most scientists surmised that GRB's originated from magnetic neutron stars residing in a thick disk in the Milky Way, about 200 to 2000 pc in thickness (Higdon and Ligenfelter 1990, Harding 1991). It was expected that the Burst and Transient Experiment (BATSE) on the CGRO would find a concentration of GRBs in the Galactice plane. Instead, the data gathered by BATSE showed that GRBs are distributed isotropically in the sky.

Given the spatially bounded, isotropic distribution, three classes of models are currently being considered. One is that bursts originate in the Oort cloud of comets that exists around the solar system. This view suffers from a lack of any appealing physical burst mechanism and the likelihood that the Oort cloud is not highly spherical.

A second class of models places neutron stars in an extended Galactic halo which would need to reach one fourth or more of the distance to the Andromeda (d = 690 kpc) in order to avoid any discernible anisotropy (Hartmann et al 1994). The neutron stars would be at these large distances because they were either born there or because they were born in the disk with high velocities and consequently migrated to their present position. The energy sources that are discussed for GRBs coming from a Galactic halo are similar to those proposed earlier for GRBs coming from a thick Galactic disk, but scaled up by 2 to 4 orders of magnitude. These would be: accretion of planetesimals, strong starquakes, thermonuclear explosions, and so on.

A third class of models, which has gained ground largely due to the BATSE results, places the GRBs at cosmological distances, perhaps at redshifts z~1-2. A source population at such a large distance naturally produces an isotropic distribution of bursts in the sky. If GRB sources lie at such great distances and radiate isotropically, tehy emit about 1e51 ergs in gamma-rays. The only know source of such a large amount of energy is gravitational collapse. Hence, all currently favored models of this class invoke either the formation of a black hole and a transitory accretion disk (e.g., the coalescence of two neutron stars in a close binary or a "failed" supernova), or the accretion of a star into a pre-existing massive black hole.

One fundamental determination that will help decide which of these models accurately describes GRBs is their precise location. The current observed error boxes are still too large for these phenomena to be associated with any object detected in other parts of the electromagnetic spectrum, such as in the optical range. Any such linkage with a quiescent, readily observable counterpart paves the way for follow on studies and measuremnets in other bands.