Photometric and Spectroscopic Observations of GRB 140629A

TL;DR

This paper reports optical and spectroscopic observations of GRB 140629A, determining its redshift, analyzing its afterglow with standard models, and discussing implications for GRB ejecta composition and empirical relations.

Contribution

The study provides detailed observational data and analysis of GRB 140629A, including its redshift, afterglow modeling, and implications for GRB ejecta and empirical correlations, highlighting potential selection effects.

Findings

Redshift of z=2.275 was measured.

Optical light curve fits standard forward shock models.

GRB radiating efficiency is approximately 0.24%.

Abstract

We present our optical photometric and spectroscopical observations of GRB 140629A. A redshift of $z=2.275\pm0.043$ is measured through the metal absorption lines in our spectroscopic data. Using our photometric data and multiple observational data observed with other telescopes, we show that its optical light curve is well interpreted with the standard forward shock models in the thin shell case. Its optical-X-ray afterglow spectrum is jointly fitted with a single power-law function, yielding a photon index of $-1.90\pm 0.05$. The optical extinction and neutral hydrogen absorption of the GRB host galaxy are negligible. The fit to the light curve with the standard models shows that the ambient density is $60\pm 9$ cm$^{-3}$ and the GRB radiating efficiency is as low as $\sim 0.24\%$, likely indicating a baryonic-dominated ejecta of this GRB. This burst agrees well with the $L_{\rm p, iso}-E_p^{'}-\Gamma_0$ relation, but confidently violates those empirical relations involving geometric corrections (or jet break time). This gives rise to an issue of possible selection effect on these relations since the jet opening angle of this GRB is extremely narrow (0.04 rad).