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 determination of the long-sought GRB luminosity function.Since BeppoSAX does not detect short bursts, we have no idea of the nature or even existence of optical, X-ray, or radio afterglows for these objects. Swift will be sensitive to the shortest events, and will provide far better coverage of these events than has been possible.
In the interesting scenario that Swift discovers some GRBs with no X-ray or UV/optical afterglow, the BAT will still provide positions of 1–4 arcmin, which is sufficient to look for radio or IR counterparts. Only the rapid response of Swift will allow identification of such a new and unusual sub-class of GRB event.
If there is a sub-class of GRBs that is the signal of conventional supernova explosions, the UVOT will provide unique and unprecedented coverage of the optical and UV light curve during the early stage.
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