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

TL;DR

This paper analyzes time-resolved observations of GRB110721A, revealing variable jet properties such as decreasing Lorentz factor and increasing nozzle radius, challenging existing models and explaining observed spectral features.

Contribution

It provides the first detailed measurement of jet property evolution during a GRB using photospheric emission, highlighting significant deviations from standard models.

Findings

Lorentz factor decreases from ~1000 to ~150 during the burst

Flow nozzle radius increases from 10^6 cm to 10^9 cm

Jet properties explain the broken power-law temperature decay

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 $\Gamma \sim 1000$ 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.