Towards the understanding of the paucity of GRB detections with Fermi/LAT: measurement of a spectral cutoff in the bright GRB 100724B

Author: Giacomo Vianello

Abstract: We will report about the detection, for the first time, of a curvature around 30 MeV in the spectrum of the prompt emission of GRB 100724B, which can be well modeled by adding an exponential cutoff to the high-energy power law of the Band model. The presence of such spectral cutoffs has been hypothesized by previous works to explain the paucity of detections from Fermi/LAT. Such measurement has been recently made possible by the introduction of a new class of Fermi/LAT data, the LAT Low Energy class (LLE), which covers the energy range 30 MeV - 100 MeV and fills the gap between the Fermi/GBM BGO detectors (covering up to few tens of MeV) and the LAT standard classes covering above 100 MeV. We will show that the detection of the feature is statistically highly significant and, moreover, cannot be explained by the systematic uncertainties on the response of the instrument, as estimated through an extensive calibration work. In addition, the large effective area of the LLE class and the resulting high photon statistic allow us to measure, for the first time, the variability time scale for this burst in the same energy range in which we measure the cutoff. Interpreting the cutoff as due to pair production, and using the minimum variability time scale, we provide an estimate for the bulk Lorentz factor of the jet responsible for the GRB.