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Asaf Pe'er

## Implications of identifying thermal component in GRB spectra

Motivated by the need to understand the origin of GRB prompt emission, it was shown by Ryde that in numerous GRBs, the spectra can be fitted with a Planck function, accompanied by an adidtional power law (non-thermal) component. This idea was further extended by Ryde & Pe'er, who were able to identify sucha component in a large sample of BATSE bursts. Moreover, when looking at the temporal evolution of both the temperature and flux of the thermal component, a repetitive behaviour was found: the thermal flux first rises reaches a peak after few seconds, and then decays as a power law as t^{-2}. Similarly, the temperature decays in time at T~t^{-2/3}. This repetitive behavious is the basis of identification of thermal component, which is now found in many bursts seen by Fermi sattelite.

From a theoretical perspective, such a (quasi)-Planck function is expected from jets viewed on-axis (or close to it). The late time decays of the temperature and flux can be naturrally explained if these photons originate from off-axis regions; this was demonstrated in my 2008 paper.

If this interpretation is correct, it has a very strong implication. Within the context of GRB "fireball" model, the photosphere depends on only 3 parameters: the total luminosity, L, the bulk motion Lorentz factor, Γ, and the initial expansion radius, r_0. Thus, for bursts with known red-shift, there are three measurable quantities (temperature, flux and distance), and three unknowns of the dynamics (initial expansion radius, bulk motion Lorentz factor, and luminosity [or photospheric radius]), which can therefore be determined. Using simple algebra, one can calculate the dynamical variables from the unknowns; this gives a direct measurement of the fireball dynamics. Since the initial expansion radius (r_0) is likely related to the Schwarzshield radius of the star, and the bulk Lorentz factor relates to the mass ejection rate, this method enables to put strong constraints on GRB progenitors.

On the other hand, since thermal component is expected in the context of the GRB fireball model, lack of such a component could indicate that this model needs to be modified; e.g., existance of strong magnetic field can reduce the thermal component.

##### Selected Publications

Pe'er, A., Ryde, F., Wijers, R., Meszaros, P., & Rees, M.J. (2007), "A New Method of Determining the Initial Size and Lorentz Factor of Gamma-Ray Burst Fireballs Using a Thermal Emission Component" Ap.J., 664, L1
Ryde, F., & Pe'er, A. (2009), "Quasi-blackbody Component and Radiative Efficiency of the Prompt Emission of Gamma-ray Bursts" Ap.J., 702, 1211
Zhang, B., & Pe'er, A. et. al. (2009), "Evidence of an Initially Magnetically Dominated Outflow in GRB 080916C" Ap.J., 700, L65
Ryde, F., & Pe'er, A. et. al. (2011), "Observational evidence of dissipative photospheres in gamma-ray bursts" MNRAS, 415, 3693