T. Nymark

Observational evidence of dissipative photospheres in gamma-ray bursts

The emission from a gamma-ray burst (GRB) photosphere can give rise to a variety of spectral shapes. The spectrum can retain the shape of a Planck function or it can be broadened and have the shape ...

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

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 outow. We nd a strong evolution in the ow properties: the Lorentz factor decreases with time during the bursts from 1000 to 150 (assuming a redshift z = 2; the values are only weakly dependent on unknown eciency parameters). Such a decrease is contrary to the expectations from the internal shocks and the isolated magnetar birth models. Moreover, the position of the ow nozzle measured from the central engine, r0, increases by more than two orders of magnitude. Assuming a moderately magnetised outow we estimate that r0 varies from 10 6 cm to 10 9 cm during the burst. We suggest that the maximal value reects 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.

Confronting GRB prompt emission with a model for subphotospheric dissipation

The origin of the prompt emission in gamma-ray bursts (GRBs) is still an unsolved problem and several different mechanisms have been suggested. Here, we fit Fermi GRB data with a photospheric emission model which includes dissipation of the jet kinetic energy below the photosphere. The resulting spectra are dominated by Comptonization and contain no significant contribution from synchrotron radiation. In order to fit to the data, we span a physically motivated part of the models parameter space and create DREAM (Dissipation with Radiative Emission as A table Model), a table model for XSPEC. We show that this model can describe different kinds of GRB spectra, including GRB 090618, representing a typical Band function spectrum, and GRB 100724B, illustrating a double peaked spectrum, previously fitted with a Band+blackbody model, suggesting they originate from a similar scenario. We suggest that the main difference between these two types of bursts is the optical depth at the dissipation site.