In about a dozen cases an optical transient following a gamma-ray burst is superimposed on a persistent, sometimes extended, source which might be the host galaxy. In four cases the spectrum of the persistent source puts the source at cosmological distances. Therefore bursts are cosmological, the mystery has been solved, and the study of this phenomenon can fade into obscurity as yet another subfield of astrophysics. But has the mystery really been solved?
We still are uncertain about the bursts' environment and distance scale. Further, the conclusion that the observed emission results from a relativistic fireball is based on the large energy which is released in a small volume; few observational signatures of such a fireball have been identified in bursts' spectral and temporal behavior. Specifically, we do not know whether the observed gamma-ray radiation results from synchrotron, inverse Compton or some other emission mechanism. The observed spectral evolution is unexplained, particularly the softening of successive intensity spikes. In addition, even if a relativistic fireball produces the observed emission, the ultimate energy source is unknown since the fireball erases almost all memory of its origin. The merger of a neutron star-neutron star binary has been proposed as the energy source, but the admittedly incomplete calculations carried out to date do not verify the favored scenario. If bursts originate at higher redshifts than implied by the minimal cosmological model, then the energy requirements may exceed the output of the merger of solar mass scale objects (the angular extent of the gamma-ray emission and therefore the total energy radiated are unknown). Consequently, other energy sources have been suggested, such as the supernovae of massive stars. Hence the origin of the bursts' energy is still a mystery.
The where, how and why of the burst phenomenon remain uncertain. Further, it is clear that bursts involve extreme physics: the release of a large energy in a small volume on a short timescale, resulting in a relativistic fireball, possibly entraining substantial magnetic fields. Finally, a deeper understanding of the origin of bursts may require the history of matter on cosmological timescales; for example these events may trace the starbursts accompanying galaxy formation. Therefore, the study of gamma-ray bursts will remain an exciting and lively field for the foreseeable future.