Non-detection of the Gamma-ray Burst X-ray Emission Line: The Down-Comptonization Effect

TL;DR

This paper investigates why gamma-ray burst X-ray emission lines are often not detected, modeling the down-Comptonization process to explain the line's diminishing intensity and shape changes over time.

Contribution

It introduces a detailed model of the time evolution of GRB X-ray emission lines considering down-Comptonization effects, explaining the non-detection of these lines.

Findings

Line intensity decreases over time depending on electron density and temperature.

High electron density leads to rapid line suppression within 100 seconds.

Line profile evolves from Gaussian to blackbody shape at thermal equilibrium.

Abstract

The detection of the gamma-ray burst (GRB) X-ray emission line is important for studying the GRB physics and constraining the GRB redshift. Since the line-like feature in the GRB X-ray spectrum was first reported in 1999, several works on line searching have been published over the past two decades. Even though some observations on the X-ray line-like feature were performed, the significance remains controversial to date. In this paper, we utilize the down-Comptonization mechanism and present the time evolution of the Fe K$\alpha$ line emitted near the GRB central engine. The line intensity decreases with the evolution time, and the time evolution depends on the the electron density and the electron temperature. In addition, the initial line with a larger broadening decreases less over time. For instance, when the emission line penetrates material with the an electron density above $10^{12}$ cm$^{-3}$ at 1 keV, it generally becomes insignificant enough after 100 s for it not to be detected. The line-like profile deviates from the Gaussian form, and it finally changes to be similar to a blackbody shape at the time of the thermal equilibrium between the line photons and the surrounding material.