Are There DIfferent Classes of Bursts with Different Physical Processes at Work?
Swift will determine whether sub-classes of GRBs exist and what fundamental differences in the source physics cause the classes. While increasing evidence of at least two sub-classes has been obtained (e.g., bimodal duration distribution, different temporal morphologies, possible correlation of hardness, and logN-logP shape, short-bursts having V/Vmax consistent with a Euclidean distribution) it is not clear if these are real differences in physical phenomena or simply represent the distribution function of GRB properties such as beaming angle, density of the local medium, or initial energy injection. Swift data will determine locations, redshift distributions, and afterglow properties of the different classes and thus allow physical understanding of their existence and/or nature. Central to the confusion regarding potential classes is that we do not have a reliable standard candle. Swift remedies that by directly measuring the distance through redshift and will make an exact determina
There is strong evidence that there are two sub-classes of gamma-ray bursts. The majority of gamma-ray bursts have durations longer than about two seconds and emit most strongly at lower energies. These are the long-soft bursts. A second, smaller sub-class is the short-hard bursts, which have durations less than about two seconds and emit most strongly at higher energies. Swift detects both classes of gamma-ray burst and has been instrumental is measuring their intrinsic properties and determining their progenitors. Swift data has determined the locations, redshift distributions, and afterglow properties of both long-soft and short-hard bursts, allowing a physical understanding of their properties. Swift has detected more than 45 short-hard bursts and found X-ray afterglows for >70% of them. More than half of these bursts have host galaxy identifications or redshift estimates. The distribution of short-hard bursts within their host galaxies is consistent with these bursta being caused
