These were two separate talks, but it is fair to say that the subject probably deserves twice as much attention; Gamma-Ray Bursters are among the most mysterious phenomena to have emerged from astrophysics in the past few years, and are certainly the most powerful! Because of the nature of these often very faint and short-lived glows, they are better suited to CCD observers than those of us who insist on using just our eyes.
GRB's were first noticed during the cold war, when each 'side' thought they were results of experiments by the other. When it became clear that this was not the case, due to the distances involved, various theories were put forward to explain them. Localisation has become increasingly remote. At first they were thought to reside within the Milky Way, but since they are observed at all galactic latitudes, this cannot be the case. They are characterised by these criteria:
Since GRBs are so short-lived, it is hard to derive accurate positions for them, and the rapid speed of appearance and variation tends to suggest small angular, if not necessarily physical, size (look at quasars, for example). This all leads to difficulties when we come to try and explain exactly what they are, always assuming of course, that they all have the same cause! If we assume matter moving at relativistic speeds, certain facts become easier to fit in with models, but then we need to find causes for such high speeds. Several models end by postulating a final result of a Black Hole plus Accretion Disk:
One curious fact about GRB's is that they frequently leave afterglows. These could be caused by ejecta encountering ambient gas (rather in the same way that some novae illuminate any nebulae which happen to be in their vicinity as did nova GK Persei in 1901). The gamma-ray emission itself is probably caused by collisions between ejected shells of gaseous material, which carries away the greater part of the kinetic energy.
It is difficult to create a convincing model of a Gamma-ray Burster, since we know so little about the influencing factors, for instance - what are the initial ingredients? What do we know about the ambient surroundings of the event? How does the kinetic energy of the relativistic blast wave get turned into the phenomena we actually observe? Why do the host galaxies (for there do appear to be such) tend to be blue - and so are GRB's somehow linked with these young objects where star-forming is very vigorous? Are they therefore necessarily related to the deaths of the short-lived massive stars that are formed in these regions? The wealth of question marks shows how much there is yet to be discovered about these exotic objects.