Gamma Ray Bursts vs . Afterglows

When does a GRB stop and its afterglow be-gin? A GRB may be defined as emission by internal shocks and its afterglow as emission by an external shock, but it is necessary to distinguish them observationally. With these definitions irregularly varying emission (at any frequency) must be the GRB, but smoothly varying intensity is usually afterglow. The GRB itself and its afterglow may overlap in time and in frequency, and distinguishing them will, in general, require detailed modeling. At first glance there appears to be little difficulty in distinguishing between GRB and their afterglows. GRB were discovered in 1972, and are observed in gamma-rays with detectors most sensitive to photons in the range 100– 1000 keV. The observed durations of GRB range from milliseconds to ∼ 1000 s. Afterglows were first observed in 1997 in the radio, visible and X-ray bands, and have durations of hours to months. These appear to be very different phenomena, although causally associated—afterglows follow GRB. This phenomenological distinction between GRB and their afterglows is likely to become insufficient. It is based on the observations of GRB and of afterglows by two distinct classes of instruments with distinct (and largely non-overlapping) sensitivities: 1. GRB detectors have broad angular acceptance and little sensitivity below 30 keV. Their angular acceptance implies high background levels. This makes them comparatively insensitive to steady sources of low flux, although very sensitive to transients of low fluence. These properties are not failures of instrument design; rather, they represent the optimal adaptation of detector technology to the observation of unpredictable brief transients of high peak flux but low fluence (compared to a steady source integrated over a long time). 2. Afterglows are detected with instruments which are sensitive to steady sources of low flux and known position. Observing such sources is the usual problem in Send offprint requests to: J. I. Katz astronomy, and these are conventional astronomical instruments , whose sensitivity depends on long temporal integration and high angular resolution. Their resolution discriminates against background. They cannot find unpredictable transients because their high resolution limits their angular acceptance, but once steered to an afterglow by a GRB detector they are sensitive to sources of low flux but long duration (and therefore of comparatively large fluence). This instrumental distinction between GRB and their afterglows will not survive when the gap between the two classes of instruments is bridged. To some extent, this has …