The host-galaxy response to the afterglow of GRB 100901A

TL;DR

This study analyzes the host galaxy's response to the GRB 100901A afterglow through multi-epoch spectra, revealing time-variable fine-structure lines and estimating the absorber's distance, providing new insights into GRB environments.

Contribution

First detection of the fall of Ni II metastable level population in a GRB host, with modeling estimating the absorber's distance based on UV-radiation excitation.

Findings

Detected time-variable fine-structure lines of Fe II and Ni II.

Estimated the absorber distance to be a few hundred parsecs.

No significant variation observed in intervening absorbers over time.

Abstract

For Gamma-Ray Burst 100901A, we have obtained Gemini-North and Very Large Telescope optical afterglow spectra at four epochs: one hour, one day, three days and one week after the burst, thanks to the afterglow remaining unusually bright at late times. Apart from a wealth of metal resonance lines, we also detect lines arising from fine-structure levels of the ground state of Fe II, and from metastable levels of Fe II and Ni II at the host redshift (z = 1.4084). These lines are found to vary significantly in time. The combination of the data and modelling results shows that we detect the fall of the Ni II 4 F9/2 metastable level population, which to date has not been observed. Assuming that the population of the excited states is due to the UV-radiation of the afterglow, we estimate an absorber distance of a few hundred pc. This appears to be a typical value when compared to similar studies. We detect two intervening absorbers (z = 1.3147, 1.3179). Despite the wide temporal range of the data, we do not see significant variation in the absorption lines of these two intervening systems.