Fermi Gamma-ray Space Telescope

A new fitting function for GRB MeV spectra based on the internal shock synchrotron model

Manal Yassine
(F. Piron , R. Mochkovitch , F. Daigne , F. Longo , N. Omodei , and G. Vianello on behalf of the Fermi/LAT collaboration)


The physical origin of the GRB prompt emission is still a matter of debate, despite great advances in the GRB domain. In this work, we present an investigation of the internal shock (IS) model in the context of Fermi observations. We studied the spectro-temporal evolution of the synthetic burst provided by a version of the IS model that has been implemented by F. Daigne, R. Mochkovitch at the IAP (Institut d'Astrophysique de Paris) since 1998. This model simulates the dynamics of the shocks that take place within GRB jets, as well as the synchrotron and inverse Compton radiation from a population of relativistic electrons. We simulated the synthetic burst as it would be observed by the GBM and the LAT, using their instrument response functions. We performed a detailed spectral analysis of the simulated synthetic spectra, and built a new function that is representative of their shape in the keV-MeV domain. We studied the spectral shape of 74 GRBs which are bright and fluent in the GBM. This analysis showed that the new function can fit adequately most (81%) of the MeV spectra in the sample, while the well known emperical Band function is suitable for a smaller fraction (60%). This shows that the physical IS model can reproduce most of the MeV spectra of GBM GRBs and therefore, it can explain the dissipation mechanisms within their jets.