Astronomy and Astrophysics Seminars

Photospheric emission in gamma-ray burst observations by Fermi gamma ray space telescope.

by Ms. Shabnam Iyyani (Department of Physics, KTH Royal Institute of Technology, AlbaNova University Center, Stockholm, Sweden)

Thursday, July 9, 2015 from to (Asia/Kolkata)
at TIFR ( DAA Seminar A269 )
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.