ASTRO SEMINAR

Speaker: Asaf Pe'er
Astronomical institute "Anton Pannekoek", University of Amsterdam

Place: Holcblat

Time: Thursday, 12/Apr/2007, 12:30

This talk is divided into 3 parts.

In the first part I will show that there are increasing evidence for a thermal emission component that accompanies the first seconds of the (overall non-thermal) prompt emission phase in GRBs. The temperature and flux of this thermal component show a characteristic temporal behavior for many different bursts. At the first few seconds the temperature is nearly constant, afterward it decays as a power law in time with power law index ~0.8. The ratio of the thermal Flux to the Temperature4, increases as a power law in time with power law index ~0.6-1.4.

In the second part, I will show that the temporal behavior can be understood as an extended emission from the source, taking into account the high latitude emission effect.

Finally, I will show that this interpretation allows a direct measurement of two of the least constrained parameters: the bulk motion Lorentz factor \eta, and the size at the base of the flow. This measurement is insensitive to the inherent uncertainties of previous estimates of \eta. I will present a specific example: for GRB970828 at z=0.96, \eta = 305+- 28, and r0 = (3.3+-2.1)*108 cm.