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{\it Fermi Gamma-ray Space Telescope} observations of GRB110721A haverevealed two emission components from the relativistic jet: emission from thephotosphere, peaking at $\sim 100$ keV and a non-thermal component, which peaksat $\sim 1000$ keV. We use the photospheric component to calculate theproperties of the relativistic outflow. We find a strong evolution in the flowproperties: the Lorentz factor decreases with time during the bursts from$\Gamma \sim 1000$ to $\sim 150$ (assuming a redshift $z=2$; the values areonly weakly dependent on unknown efficiency parameters). Such a decrease iscontrary to the expectations from the internal shocks and the isolated magnetarbirth models. Moreover, the position of the flow nozzle measured from thecentral engine, $r_0$, increases by more than two orders of magnitude. Assuminga moderately magnetised outflow we estimate that $r_0$ varies from $10^6$ cm to$\sim 10^9$ cm during the burst. We suggest that the maximal value reflects thesize of the progenitor core. Finally, we show that these jet propertiesnaturally explain the observed broken power-law decay of the temperature whichhas been reported as a characteristic for GRB pulses.

Variable jet properties in GRB110721A: Time resolved observations of the jet photosphere