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4 The first localization of a GRB


We have applied this method to GRB970517 (BATSE $\sharp$6235). We chose this GRB for its characteristics of intensity, morphology and relative intensities registered in different detectors: it is a strong, sharp burst lasting about 5 s, with approximately the same signal in detectors GRBM2 and GRBM3. In this preliminary attempt, we used BATSE data to derive its spectral properties (spectral index $\alpha=0.9$, assuming a power-law spectrum $\propto{E^{-\alpha}}$). As a preliminary determination of the GRB incoming direction we have only considered the ratio between the GRBM2 and GRBM3 intensities, whose dependence on the azimuth $\phi$ is shown in Fig. 1. The comparison between observed and expected ratio gives a strong selection of the possible values of $\phi$ as a function of the altitude $\theta$. The same criterion has been used to select a combination of detectors intensities giving a good selection of $\theta$ versus $\phi$. The ratio between the sum of detectors GRBM1 and GRBM4 and the sum of detectors GRBM2 and GRBM3 proved to give the best results. Both computations were carried out for the two GRBM spectral bands (GRBM band, 40-700 keV and AC band, 100-700 keV). The final error box is shown in Fig. 2 along with BATSE's position.



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