Fermi Gamma-ray Space Telescope

The correlated variation of the GRB intensity and spectral shape indicates photospheric emission

Felix Ryde
(H.-F. Yu, H. Dereli-B’egu’e, C. Lundman, A. Pe’er, L. Li)


The emission mechanism during the prompt phase in GRBs can by investigated though correlations between spectral properties. Here, we study the strong correlation relating the instantaneous flux, $F$, and the photon index below the spectral break, $alpha$, in GRB pulses observed by the Fermi/GBM detector. We find that in the 38 most prominent pulses in the GBM catalogue the correlation fits the relation $F(t) propto {rm e}^{kalpha(t)}$. We argue that this correlation is a manifestation of subphotospheric heating in a flow with a varying entropy, which naturally leads to a correlated variation of the intensity and spectral shape of the photospheric emission.