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We present time resolved spectral analysis of prompt emission from GRB160625B, one of the brightest bursts ever detected by Fermi in its nine yearsof operations. Standard empirical functions fail to provide an acceptable fitto the GBM spectral data, which instead require the addition of a low-energybreak to the fitting function. We introduce a new fitting function, called2SBPL, consisting of three smoothly connected power laws. Fitting this model tothe data, the goodness of the fits significantly improves and the spectralparameters are well constrained. We also test a spectral model that combinesnon-thermal and thermal (black body) components, but find that the 2SBPL modelis systematically favoured. The spectral evolution shows that the spectralbreak is located around $E_{\rm break}\sim$ 100 keV, while the usual $\nuF_{\nu}$ peak energy feature $E_{\rm peak}$ evolves in the 0.5-6 MeV energyrange. The slopes below and above $E_{\rm break}$ are consistent with thevalues -0.67 and -1.5, respectively, expected from synchrotron emissionproduced by a relativistic electron population with a low energy cut-off. If$E_{\rm break}$ is interpreted as the synchrotron cooling frequency, theimplied magnetic field in the emitting region is $\sim$ 10 Gauss, i.e. ordersof magnitudes smaller than the value expected for a dissipation region locatedat $\sim 10^{13-14}$ cm from the central engine. The low ratio between $E_{\rmpeak}$ and $E_{\rm break}$ implies that the radiative cooling is incomplete,contrary to what is expected in strongly magnetized and compact emittingregions.

Consistency with synchrotron emission in the bright GRB 160625B observed by Fermi