The First 30 Years 2411 Discovery and BATSE

Gamma-ray bursts (GRBs) are short bursts of high-energy radiation from an unpredictable location in the sky. The y-ray emission can rise to maximum intensity within fractions of a millisecond. During these short times GRBs are the brightest objects in the y-ray sky. The energy spectra are nonthermal, with most of the power radiated in the 100-500 keV range, but photons up to 18GeV or down to a few kiloelectronvolt have also been registered. The bursts have durations of typically 0.1-100 s, with a bimodal distribution separating "short" and "long" duration GRBs at ~1-2s [57] with a relative occurrence of 1:4 [49]. In addition, short bursts are typically harder than the long bursts, supporting the conjecture that they form two classes of object (Fig. 24.1).

GRBs were first detected in 1967 with small y-ray detectors onboard the Vela satellites [46], which were designed to verify the nuclear test ban treaty between the USA and Russia. For many years the prevailing opinion was that magnetic neutron stars (NS) in the Galactic disk were the sources of GRBs [78]. No flaring emission outside the y-ray region could be detected, and no indisputable quiescent counterpart to a GRB could be established. Despite a distance "uncertainty" of 10 orders of magnitude, numerous theories (see [64] for a compilation) were advanced to explain the source of energy in GRBs. The measurements since 1991 of the burst and transient source experiment (BATSE) onboard the Compton gamma-ray observatory have shown unequivocally that GRBs are isotropically distributed on the sky (Fig. 24.2) even at the faintest intensities, and that there is a distinct lack of faint bursts as compared to a homogeneous distribution in Euclidian space [58]. This first indicated a cosmological origin, though also a distribution in the halo of our Galaxy was considered. The y-ray spectra are well described by broken power laws, with mean slopes of a = —1 at low energies (10-300keV) and of P = -2.4 at high energies [70]. Consequently, the bulk of the emission is radiated at energies around the spectral turnover £break, more specifically at the peak energy Epeak = £break(2 + a). An unprecedented wealth of additional information on each burst as well as a number of population properties could be collected, yet the GRB origin remained a mystery.

Fig. 24.1 Hardness of the prompt y-ray emission as a function of duration T90 for all GRBs of the 4th BATSE catalog, showing the two classes of short/hard and long/soft GRBs as first demonstrated in 1993 [49]. T90 is the time interval over which a burst emits from 5% to 95% of its total measured flux, and the hardness ratio HR23 is defined as (2 — 3)/(2 + 3), where "2" stands for the flux in the 50-100keV band, and "3" for that in the 100-300keV band, respectively. Spectrally soft GRBs have a large HR23

Fig. 24.1 Hardness of the prompt y-ray emission as a function of duration T90 for all GRBs of the 4th BATSE catalog, showing the two classes of short/hard and long/soft GRBs as first demonstrated in 1993 [49]. T90 is the time interval over which a burst emits from 5% to 95% of its total measured flux, and the hardness ratio HR23 is defined as (2 — 3)/(2 + 3), where "2" stands for the flux in the 50-100keV band, and "3" for that in the 100-300keV band, respectively. Spectrally soft GRBs have a large HR23

Fig. 24.2 Sky distribution of 2704 GRBs as measured by BATSE during the full gamma-ray observatory mission lifetime from 1991 to 2000 (derived from http://f64.nsstc.nasa.gov/batse/grb/skymap.)

Fig. 24.2 Sky distribution of 2704 GRBs as measured by BATSE during the full gamma-ray observatory mission lifetime from 1991 to 2000 (derived from http://f64.nsstc.nasa.gov/batse/grb/skymap.)

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