Multi-Component Spectral Analysis of Bright Gamma Ray Bursts observed with the Fermi Gamma ray Space Telescope

Author: Sylvain Guiriec

Abstract: The recent observations of Gamma-Ray Bursts (GRBs) with the Gamma-ray Burst Monitor (GBM) and the Large Area Telescope (LAT) onboard the Fermi Gamma Ray Space Telescope, open a new window in the understanding of their prompt emission. With data sets from instruments prior to Fermi, GRB prompt emission spectra in the keV-MeV energy range were adequately fit with the empirical Band function, which consists of two power laws (PLs) smoothly connected at a break energy. The Band function is usually associated to nonthermal emission processes. Spectral analysis over the broad energy range of GBM shows deviations from this function. These deviations are sometimes adequatelly fit with an additional PL extending from the lowest energy in GBM up to few tens of GeV in the LAT. We also recently clearly identified, for the first time, a physical thermal spectral component together with the non-thermal one. We present here the identification of multiple spectral components in the GBM prompt emission spectra of some bright GRBs: a photospheric thermal component, a broken PL most likely associated with synchrotron emission from electrons propagating in the GRB jet, and an additional PL. Using time integrated and detailed time-resolved spectroscopy of GBM data, we show the temporal evolution of the various spectral components and their relative contributions. We will see that it is possible to associate the various spectral components with light curve structures in various energy bands in GBM and LAT. Multi-component fits allow better constraints on the GRB prompt emission spectral shape. Therefore, this new approach can reconcile the observations with the models which were challenging the spectral parameters of the Band function. We will discuss the interpretation of the various components in terms of emission mechanisms and acceleration processes, and we will examine the consequences on the central engine and jet properties.