Fine-Structure FeII and SiII Absorption in the Spectrum of GRB 051111: Implications for the Burst Environment

TL;DR

This study analyzes fine-structure FeII and SiII absorption in GRB 051111's spectrum to understand the burst environment, suggesting it is a star-forming region influenced by the GRB's radiation, with implications for high-redshift star formation.

Contribution

It introduces detailed analysis of fine-structure absorption lines in GRB afterglow spectra to probe the local environment and excitation mechanisms, highlighting the role of the GRB's radiation in excitation processes.

Findings

Fine-structure lines confined to a narrow velocity range, indicating a local environment.

Radiative pumping by the GRB's radiation is a plausible excitation mechanism.

The environment likely contains >10^3 OB stars within a few parsecs.

Abstract

We present an analysis of fine-structure transitions of FeII and SiII detected in a high-resolution optical spectrum of the afterglow of GRB 051111 (z=1.54948). The fine-structure absorption features arising from FeII* to FeII****, as well as SiII*, are confined to a narrow velocity structure extending over +/-30 km/s, which we interpret as the burst local environment, most likely a star forming region. We investigate two scenarios for the excitation of the fine-structure levels by collisions with electrons and radiative pumping by an infra-red or ultra-violet radiation field produced by intense star formation in the GRB environment, or by the GRB afterglow itself. We find that the conditions required for collisional excitation of FeII fine-structure states cannot be easily reconciled with the relatively weak SiII* absorption. Radiative pumping by either IR or UV emission requires >10^3 massive hot OB stars within a compact star-forming region a few pc in size, and in the case of IR pumping a large dust content. On the other hand, it is possible that the GRB itself provides the source of IR and/or UV radiation, in which case we estimate that the excitation takes place at a distance of ~10-20 pc from the burst. Detailed radiative transfer calculations are required in order to verify that excitation of the low-ionization fine-structure states is possible given the intense UV flux from the burst. Still, it is clear that GRB absorption spectroscopy can provide direct information on the mode and conditions of star formation at high redshift.