Constraining the bulk Lorentz factor from the photosphere emission

Abstract

We propose a direct and model-independent method to constrain the Lorentz factor of a relativistically expanding object, like gamma-ray bursts. Only the measurements, such as thermal component of the emission, the distance and the variable time scale of the light curve, are used. If the uncertainties are considered, we will obtain lower limits of the Lorentz factor instead. We apply this method to GRB 090618 and get a lower limit of the Lorentz factor to be 22. The method can be used to any relativistically moving object, such as gamma-ray bursts, blazars, and soft gamma-ray repeaters, providing the thermal component of the emission being observed.

arXiv comment10 pages, 1 figure

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• Characteristics of Long Gamma-Ray bursts in the Comoving Frame
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• Detection of Prompt Fast-Variable Thermal Component in Multi-Pulse Short Gamma-Ray Burst 170206A
• Jets in a Gamma-Ray Burst During its Prompt Emission: Evolution of Lorentz Factor
• $L_{\mathrm{syn}}-E_{\mathrm{syn},\mathrm{p}}-\delta$ relation in Active Galactic Nucleus Jets and Implication for the physical origin of the $L_{\mathrm{p}}-E_{\mathrm{p},\mathrm{z}}-{\Gamma }_0$ relation of Gamma-Ray Bursts