Is magnetically dominated outflow required to explain GRBs?

Abstract

The composition of relativistic outflows producing gamma-ray bursts is a long-standing open question. One of the main arguments in favour of magnetically dominated outflows is the absence of photospheric component in their broad-band time-resolved spectra, with such notable example as GRB 080916C. Here, we perform a time-resolved analysis of this burst and confirm the previous detection of an additional spectral component. We show that this subdominant component is consistent with the photosphere of ultrarelativistic baryonic outflow, deep in the coasting regime. We argue that, contrary to previous statements, the magnetic dominance of the outflow is not required for the interpretation of this GRB. Moreover, simultaneous detection of high-energy emission in its prompt phase requires departure from a one-zone emission model.