Gamma-ray bursts are transient events which originate beyond the solar system. Emission has been observed during the actual burst only above ~1 keV; recently lower energy afterglows have been detected. Bursts constitute a very diverse population, with properties characterized by large dynamic ranges. Currently two types of bursts have been identified: classical bursts and Soft Gamma Repeaters (SGRs). Note that SGRs are identified by their galactic coordinates, while bursts are named by the date of their occurrence, GRB yymmdd.
Four repeating SGRs are currently known. One appears to be in the Large Magellanic Cloud (LMC) while the other three are towards the center of our galaxy. Their spectra can be described as optically-thin Bremsstrahlung with a temperature of ~25-40 keV, and are therefore softer (i.e., lower average photon energy) than the spectra of classical bursts (as will be discussed below). The most extraordinary burst from an SGR was the first observed event from SGR 0525-66, which occurred on March 5, 1979; this burst began with an intense spike and concluded with 25 cycles of an 8 second period. This SGR has been localized to the supernova remnant N49 in the LMC. Similarly SGR 1806-20 is coincident with a supernova remnant in our Galactic plane. On the other hand, SGR 1900+14 is outside of a supernova remnant; there are no obvious supernova remnants in the error box of the newly discovered SGR 1815-14. Based on this small sample, an evolutionary scenario has been suggested in which SGRs are high velocity neutron stars which eventually escape the supernova remnant created by the supernova in which the neutron star was born. While SGRs are interesting in their own right, here I focus on classical bursts.
Classical bursts are known to range in duration from 4 ms to more than 1000 s; the duration distribution is bimodal, with a cusp at a duration of 2 s. The broad spectrum usually peaks between a few 100 keV and a few MeV; generally most of the energy is emitted around an MeV, with only a few percent in the X-ray band, although some events are rich in X-ray emission. As will be discussed below, the existence of line features in burst spectra is controversial. The light curves vary in appearance: some bursts are very smooth, while some are very spiky. Only one class has been defined: about 15% of all bursts are FREDs---Fast Rise, Exponential Decay. Integrating over the spectrum, the peak photon flux ranges between 0.1 and 100 photons cm-2 s-1, and the energy fluence has been observed between 10-7 and 10-3 erg cm-2. Of course, a quantity's observed distribution is a convolution of the true distribution and a given detector's characteristics. For example, BATSE (the largest detector system flown thus far) detects bursts by an increase in the counts accumulated in 64, 256 and 1024 ms bins; consequently BATSE becomes progressively more insensitive as the burst duration decreases below 64 ms. Nonetheless, it is clear that burst properties are characterized by large dynamic ranges.
Bursts are interesting for many reasons. Of course, their unknown origin challenges and entices us to solve the mystery. Furthermore, the phenomenon must involve exotic physics. Somehow gamma-rays are emitted efficiently with little low energy radiation, at least during the burst. The rapid time variability (on scales of less than a millisecond) indicates a small source size (tens of kilometers) yet the large distance to the source requires a large energy release (more than 1051 ergs if radiated isotropically at cosmological distances); consequently, the energy density must be enormous. Finally, if bursts originate in distant galaxies, as appears to be the case, then they may probe the evolution of the universe. Burst repetitions and temporal structure within bursts may reveal gravitational lensing by intervening structure. Bursts may trace the star formation history of galaxies soon after they formed. Thus cosmological bursts are intimately tied to the evolution of matter in the universe. Therefore, the allure of gamma-ray bursts should not be diminished by the conclusion that they are cosmological as opposed to Galactic.