The combination of very high energy (VHE; E > 100 GeV) and high energy (MeV to GeV) gamma-ray Based on the experience gained during the first phase of the mission, a radical revision of the Fermi Large Area Telescope event level analysis has been ongoing within the collaboration. This revision affects the entire analysis scheme, from the Monte Carlo simulations to event reconstruction and background rejection. Although still under development, we can effectively benchmark the new event reconstruction in the special case of the prompt phase of bright Gamma-Ray Bursts (GRBs), where the signal to noise is large enough that loose selection cuts are sufficient to identify gamma-rays associated with the source.
Using the new tracker reconstruction and calorimeter clustering algorithms, we have re-analyzed eight GRBs previously detected by the LAT for which an x-ray/optical follow-up was possible and found four new photons with energies greater than 10 GeV in addition to the six previously known. Among these four is a ~29 GeV photon from GRB080916C, which has a redshift of 4.35, thus making it the highest intrinsic energy (~155 GeV) detected from a GRB. We present here the salient aspects of the new event reconstruction and discuss the scientific implications of these new high energy photons, such as constraining some extragalactic background light (EBL) models, the prompt emission mechanism and the bulk Lorentz factor of the emitting region.