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

Shabnam Iyyani; Felix Ryde; Magnus Axelsson; James Michael Burgess; Sylvain Guiriec; Josefin Larsson; Christoffer Lundman; Elena Moretti; Sinead McGlynn; Tanja Nymark; Kjell Rosquist

doi:10.1093/mnras/stt863

paper

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

arXiv:1305.3611 · doi:10.1093/mnras/stt863

Abstract

{\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 $Γ \sim 1000$ 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 $1 0^{6}$ cm to $\sim 1 0^{9}$ 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.

Contains 11 pages and 9 figures; Accepted for publication in MNRAS

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

Abstract

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