A Global Photoionization Response to Prompt Emission and Outliers: Different Origin of Long Gamma-ray Bursts?

TL;DR

This study investigates how the ionization state of the interstellar medium around long gamma-ray bursts correlates with their prompt emission properties, revealing potential different origins for outliers and typical bursts.

Contribution

It introduces a global photoionization response model linking ISM ionization ratios to gamma-ray burst prompt emission parameters, suggesting different progenitors for outliers.

Findings

Ionization ratio increases with $L_{iso}/E_{peak}$ and $L_{iso}/E_{peak}^2$ for most LGRBs.

Outliers show high ionization ratios and Wolf-Rayet star signatures.

Scatter in the Amati relation may be related to ISM ionization states.

Abstract

By using the line ratio \ion{C}{4}$\lambda1549$/\ion{C}{2}$\lambda1335$ as a tracer of ionization ratio of the interstellar medium (ISM) illuminated by a long gamma-ray burst (LGRB), we identify a global photoionization response of the ionization ratio to the photon luminosity of the prompt emission assessed by either $L_{\mathrm{iso}}/E_{\mathrm{peak}}$ or $L_{\mathrm{iso}}/E^2_{\mathrm{peak}}$. The ionization ratio increases with both $L_{\mathrm{iso}}/E_{\mathrm{peak}}$ and $L_{\mathrm{iso}}/E^2_{\mathrm{peak}}$ for a majority of the LGRBs in our sample, although there are a few outliers. The identified dependence of \ion{C}{4}/\ion{C}{2} on $L_{\mathrm{iso}}/E^2_{\mathrm{peak}}$ suggests that the scatter of the widely accepted Amati relation is related with the ionization ratio in ISM. The outliers tend to have relatively high \ion{C}{4}/\ion{C}{2} values as well as relatively high \ion{C}{4}$\lambda1549$/\ion{Si}{4}$\lambda1403$ ratios, which suggests an existence of Wolf-Rayet stars in the environment of these LGRBs. We finally argue that the outliers and the LGRBs following the identified \ion{C}{4}/\ion{C}{2}$-L_{\mathrm{iso}}/E_{\mathrm{peak}}$ ($L_{\mathrm{iso}}/E^2_{\mathrm{peak}}$) correlation might come from different progenitors with different local environments.