Author: Suzanne Foley
Abstract: The Fermi Gamma-ray Burst Monitor (GBM) has detected over 1000 GRBs since its launch in June 2008. Many GRBs exhibit a spectral lag up to ∼1 MeV which is seen as high-energy gamma-ray emission arriving earlier than photons in a low-energy band. There is also evidence that the high-energy emission in the GeV energy range, as detected by the Large Area Telescope, is delayed relative to that below 1 MeV for some short and long bursts. Here we present the spectral lags of a sample of bright and hard GRBs detected by GBM, utilising data from the 12 NaI detectors and 2 BGO detectors. The wide energy coverage of GBM (8 keV - 40 MeV) allows the dependence of the spectral lag on energies up to the MeV range to be investigated for bursts with sufficient signal in the BGO detectors. The relationship between the evolution of the spectral lag with energy and the spectral evolution of the burst is also investigated.