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tokens: time interval, covering up to 15 ks after the beginning of the prompt emission, and adopt E min = 1 GeV to recover exposure. The condition in Equation ( 20 ) now returns 10,962 s of GTIs in the first 15 ks after t 0 . In this case, six events survive the cuts, among which a ∼10 GeV photon at t 0 + 2239 s. Note that there are still no photons during the prompt emission with this selection. We detect and localize the source producing a TS map for this time interval, with a likelihood model containing the appropriate Galactic and isotropic templates provided by the LAT collaboration as well as all the point sources from the 3FGL catalog (Acero et al. 2015 ). We model GRB 100724B as a point source with a power-law spectrum, and we keep the parameters for all 3FGL sources fixed to their catalog values. No 3FGL source is detected in the small time window of our analysis, thus this choice is irrelevant for our final results. This analysis results in a firm detection of the GRB with a significance of ∼7 σ . The best-fit power law has a photon index of α = −1.8 ± 0.4, a value typical for the high-energy emission of GRBs (Ackermann et al. 2013b ), with an average flux of erg cm −2 s −1 (1–100 GeV). Our localization is shown in Figure 15 . The cross marks the best-fit position, corresponding to R.A. = 12347 and decl. = 7588 (J2000), while the white lines correspond to the 68% and 90% c.l. containment regions. We used this position in the following analysis. The most accurate localization available in the literature before this work was reported in the Fermi /LAT GRB catalog (Ackermann et al. 2013b ), and corresponds to R.A. = 11959, decl. = 7586 (J2000) with a 68% containment radius of 088 (dashed line in Figure 15 ). This localization was based on the detection during the prompt phase, obtained by relaxing the zenith angle cut using a threshold of , and it is compatible with the one found in this work, but it features a much larger containment radius. The excess we find is not related to any known source and lies within the GBM localization region for GRB 100724B. The source is not detected in any other time interval. We therefore identify it as the high-energy counterpart of the burst. Figure 15. Localization of GRB 100724B: the white contours are, respectively, the 68% and 90% containment regions, while the dotted line is the margin of the much larger 68% containment region provided in Ackermann et al. ( 2013b ) based on the prompt interval. The black cross is the best-fit localization. Appendix B Likelihood Ratio Test The Likelihood Ratio Test (LRT) is a procedure for model selection. It is a statistical test that helps to select between two nested models, m 0 and m 1 , the one that best captures all significant features of the data. The model m 0 is the null hypothesis and represents the simpler model, while m 1 is the alternative hypothesis and is more complex than m 0 . The TS is twice the difference in the log-likelihood S between the two