2018ApJ...855...91W

By using the line ratio C IV λ1549/C IIλ1335 as a tracer of the 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_iso/E_peak or L_iso/E_peak². The ionization ratio increases with both L_iso/E_peak and L_iso/E²_peak for a majority of the LGRBs in our sample, although there are a few outliers. The identified dependence of C IV/C II on L_iso/E_peak² suggests that the scatter of the widely accepted Amati relation is related to the ionization ratio in the ISM. The outliers tend to have relatively high C IV/C II values as well as relatively high C IV λ1549/Si IV λ1403 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 C IV/C II-L_iso/E_peak (L_iso/E_peak²) correlation might come from different progenitors with different local environments.