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

Abstract

By using the line ratio \ion{C}{4}$\lambda 1549$/\ion{C}{2}$\lambda 1335$ 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 $L_{\mathrm{iso}}/E_{\mathrm{peak}}^2$. The ionization ratio increases with both $L_{\mathrm{iso}}/E_{\mathrm{peak}}$ and $L_{\mathrm{iso}}/E_{\mathrm{peak}}^2$ 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 $L_{\mathrm{iso}}/E_{\mathrm{peak}}^2$ 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}$\lambda 1549$/\ion{Si}{4}$\lambda 1403$ 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}$-L_{\mathrm{iso}}/E_{\mathrm{peak}}$ ($L_{\mathrm{iso}}/E_{\mathrm{peak}}^2$) correlation might come from different progenitors with different local environments.

7 pages, 3 figures and 2 tables, to be published in ApJ