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

{\it Fermi Gamma-ray Space Telescope} observations of GRB110721A have revealed two emission components from the relativistic jet: emission from the photosphere, peaking at \(\sim 100\) keV and a non-thermal component, which peaks at \(\sim 1000\) keV. We use the photospheric component to calculate the properties of the relativistic outflow. We find a strong evolution in the flow properties: the Lorentz factor decreases with time during the bursts from \(\Gamma \sim 1000\) to \(\sim 150\) (assuming a redshift \(z=2\); the values are only weakly dependent on unknown efficiency parameters). Such a decrease is contrary to the expectations from the internal shocks and the isolated magnetar birth models. Moreover, the position of the flow nozzle measured from the central engine, \(r_0\), increases by more than two orders of magnitude. Assuming a 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 the size of the progenitor core. Finally, we show that these jet properties naturally explain the observed broken power-law decay of the temperature which has been reported as a characteristic for GRB pulses.