Signs of magnetic acceleration and multizone emission in GRB 080825C

Abstract

One of the major results from the study of gamma-ray bursts with the Fermi Gamma-ray Space Telescope has been the confirmation that several emission components can be present in the energy spectrum. Here we reanalyse the spectrum of GRB 080825C using data from the Fermi LAT and GBM instruments. Although fairly weak, it is the first gamma-ray burst detected by the Fermi-LAT. We improve on the original analysis by using the LAT Low Energy (LLE) events covering the 30-00 MeV band. We find evidence of an additional component above the main emission peak (modelled using a Band function) with a significance of 3.5 $\sigma$ in 2 out of the 4 time bins. The component is well fitted by a Planck function, but shows unusual behaviour: the peak energy increases in the prompt emission phase, reaching energies of several MeV. This is the first time such a trend has been seen, and implies that the origin of this component is different from those previously detected. We suggest that the two spectral components likely arise in different regions of the outflow, and that strong constraints can be achieved by assuming one of them originates from the photosphere. The most promising model appears to be that the high-energy peak is the result of photospheric emission in a Poynting flux dominated outflow where the magnetisation increases with time.