Hints of the jet composition in gamma-ray bursts from dissipative photosphere models

We present a model for gamma-ray bursts where a dissipative photosphere provides the usual spectral peak around MeV energies accompanied by a subdominant thermal component. We treat the initial acceleration of the jet in a general way, allowing for magnetic field- and baryon dominated outflows. In this model, the GeV emission associated with GRBs observed by Fermi LAT, arises as the interaction of photospheric radiation and the shocked electrons at the deceleration radius. Through recently discovered correlations between the thermal and non-thermal peaks within individual bursts, we are able to infer whether the jet was Poynting flux or baryon dominated.