Constraining the bulk Lorentz factor from the photosphere emission

TL;DR

This paper introduces a model-independent method to estimate the minimum Lorentz factor of relativistic objects like gamma-ray bursts using observable emission features, applicable to various astrophysical phenomena.

Contribution

A new direct method to constrain the Lorentz factor from thermal emission, distance, and light curve variability, applicable without relying on detailed models.

Findings

Lower limit of Lorentz factor for GRB 090618 is 22.

Method provides a model-independent way to estimate Lorentz factors.

Applicable to various relativistic astrophysical objects.

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.