Fermi-LAT observations of GRBs with weak LAT emission

Abstract

We present the analysis results of three Gamma-Ray Bursts (GRBs) detected by the Gamma-ray Burst Monitor (GBM) and the Large Area Telescope (LAT) onboard Fermi: the two long GRB 080825C and GRB 090217, and the first short burst with GeV photons GRB 081024B. The emission from GRB 081024B observed by the LAT above 100 MeV is delayed with respect to the GBM trigger, and significantly extends after the low-energy episode. Some hints for spectral hardening was observed in this burst as well as in GRB 080825C, possibly related to a separate and harder component showing up at late times. Conversely, GRB 090217 does not exhibit any noticeable feature. Together with the other bright LAT detected bursts (e.g. GRB 080916C and GRB 090510), these observations help to classify the GRB properties and give new insight on the acceleration mechanisms responsible for their emission at the highest energies.