Variable jet properties in GRB 110721A: time resolved observations of the jet photosphere

Abstract

Fermi Gamma-ray Space Telescope observations of GRB110721A have revealed two emission components from the relativistic jet: emission from the photosphere, peaking at ∼ 100 keV and a non-thermal component, which peaks at ∼ 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 ∼ 1000 to ∼ 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 ∼ 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.

Transparency Parameters From Relativistically Expanding Outflows

“…Indeed R ph ∝ Γ −2 diss η −1 , where Γ diss is a pure dynamical term and η represents the electrons content of the outflow. Second, the computation of the initial radius of expansion assumes adiabatic cooling all the way from the radius at which the plasma has been created (or at least starts to accelerate (see, e.g., Thompson et al 2007;Iyyani et al 2013) up to the photosphere).…”

“…When the photospheric emission takes place at the coasting phase (R ph > R s ), it is obvious that increasing the value of Y only pushes the photosphere deeper into the coasting phase. In previous studies of the outflow properties from the photospheric emission, Ryde & Pe'er (2009) and Iyyani et al (2013) associate Y to the adiabatic cooling factor (R ph /R s ) −2/3 = F BB /YF. This is a good approximation when the outflow is deep in the coasting phase but is not correct at the accelerating phase as well as when the photosphere is close to R s .…”

“…Then, such properties were successfully obtained from a large number of bursts for which a thermal component was found (see, e.g., Ryde & Pe'er 2009;Iyyani et al 2013). Nevertheless, the method has three underlying assumptions: (1) the photospheric emission takes place in the coasting phase; (2) the infinite and steady wind approximation can be considered for the computation of the optical depth; and (3) there is no sub-photospheric dissipation, i.e., when the outflow is still optically thick.…”

Surprise in simplicity: an unusual spectral evolution of a single pulse GRB 151006A

“…The observed emission can be entirely from the photosphere wherein the broadness is contributed by the continuous or localised subphotospheric dissipation of the kinetic energy or Poynting flux (Beloborodov 2010(Beloborodov , 2013Giannios 2012;Pe'er, Mészáros & Rees 2005;Iyyani et al 2015), or may be due to the geometrical effects of the jet emission and its structure (Lundman, Pe'er & Ryde 2013;Beloborodov 2011;Basak & Rao 2015a). Or, the photospheric emission can be a sub-dominant component (Burgess et al 2014;Axelsson et al 2012;Iyyani et al 2013;Guiriec et al 2011Guiriec et al , 2013. All the models have certain pros and cons and confront issues in justifying the constraints obtained from observations.…”

Photospheric emission in gamma-ray bursts

“…The advantage over using just a "Band" function is evident when looking at the residuals (Taken from 91 ) There are therefore two key conclusions from these works.…”