| Description |
The large flashes of radiation that are observed in GRBs are generally believed to arise in a
relativistic jetted outflow. The talk discusses primarily my thesis work which addresses the question
of how and where in the jet this radiation is produced.
I focus on the observational effects of the emission from the jet photosphere that has been observed
by the Fermi gamma ray space telescope in the energy range 8 keV - 300 GeV. I show that the
photosphere has an important role in shaping the observed radiation spectrum and that its
manifestations can significantly vary between bursts. For bursts in which the photospheric emission
component can be identified, the dynamics of the flow can be explored by determining the jet
Lorentz factor and the position of the jet nozzle. I show that three main flow quantities can describe
the observed spectral behaviour in bursts: the luminosity, the Lorentz factor, and the nozzle radius.
While the photosphere can appear like a pure blackbody it can also be substantially broadened, due
to dissipation of the jet energy below the photosphere. I show that Comptonisation of the blackbody
can shape the observed spectra and describe its evolution. Alternative models including
synchrotron emission leads to severe physical constraints, such as the need for very high electron
Lorentz factors, which are not expected in internal shocks. Even though different manifestations of
the photospheric emission can explain the data, and lead to ambiguous interpretations, I argue that
dissipation below the photosphere is the most important process in shaping the observed spectral
shapes and evolutions. In end, I briefly discuss my future research plan.
|