A new fitting function for GRB MeV spectra based on the internal shock synchrotron model M. Yassine, F. Piron, F. Daigne, R. Mochkovitch, F. Longo, N. Omodei and G. Vianello A&A, 640 (2020) A91 Published online: 18 August 2020 DOI: 10.1051/0004-6361/201937057