Scattered Emission from A Relativistic Outflow and Its Application to Gamma-Ray Bursts

TL;DR

This paper models photon scattering in relativistic outflows with multiple shells to explain features of gamma-ray bursts and their afterglows, considering different scattering mechanisms and shell configurations.

Contribution

It introduces a simple two-shell model to analyze scattered emission in relativistic outflows, applying it to gamma-ray burst observations and exploring various scattering scenarios.

Findings

Scattered flux from cold shells is generally weak and detectable only in weak afterglow bursts.

Hot shells with large energy can produce bright scattered emission similar to X-ray shallow decay components.

Faster shells can cause late short gamma-ray or MeV flashes, or be part of the prompt emission.

Abstract

We investigate a scenario of photons scattering by electrons within a relativistic outflow. The outflow is composed of discrete shells with different speeds. One shell emits radiation for a short duration. Some of this radiation is scattered by the shell(s) behind. We calculate in a simple two-shell model the observed scattered flux density as a function of the observed primary flux density, the normalized arrival time delay between the two emission components, the Lorentz factor ratio of the two shells and the scattering shell's optical depth. Thomson scattering in a cold shell and inverse Compton scattering in a hot shell are both considered. The results of our calculations are applied to the Gamma-Ray Bursts and the afterglows. We find that the scattered flux from a cold slower shell is small and likely to be detected only for those bursts with very weak afterglows. A hot scattering shell could give rise to a scattered emission as bright as the X-ray shallow decay component detected in many bursts, on a condition that the isotropically equivalent total energy carried by the hot electrons is large, $\sim 10^{52-56}$ erg. The scattered emission from a faster shell could appear as a late short $\gamma$-ray/MeV flash or become part of the prompt emission depending on the delay of the ejection of the shell.