Spectroscopy and multiband photometry of the afterglow of intermediate duration gamma-ray burst 040924 and its host galaxy

TL;DR

This study analyzes the optical afterglow and host galaxy of gamma-ray burst 040924, providing evidence supporting its origin from a massive star core-collapse through photometry, spectroscopy, and comparison with long GRB properties.

Contribution

It offers the first detailed optical analysis of GRB 040924, linking its properties to those of long GRB hosts and confirming a massive star progenitor.

Findings

Optical afterglow fits a broken power-law with a break at ~0.03 days.

Host galaxy properties are similar to long GRB hosts.

Detected [Ne III] emission line indicates low metallicity.

Abstract

We present optical photometry and spectroscopy of the afterglow and host galaxy of gamma-ray burst 040924. This GRB had a rather short duration of T90 ~2.4s, and a well sampled optical afterglow light curve. We aim to use this dataset to find further evidence that this burst is consistent with a massive star core-collapse progenitor. We combine the afterglow data reported here with data taken from the literature and compare the host properties with survey data. We find that the global behaviour of the optical afterglow is well fit by a broken power-law, with a break at ~0.03 days. We determine the redshift z = 0.858 +/- 0.001 from the detected emission lines in our spectrum. Using the spectrum and photometry we derive global properties of the host, showing it to have similar properties to long GRB hosts. We detect the [Ne III] emission line in the spectrum, and compare the fluxes of this line of a sample of 15 long GRB host galaxies with survey data, showing the long GRB hosts to be comparable to local metal-poor emission line galaxies in their [Ne III] emission. We fit the supernova bump accompanying this burst, and find that it is similar to other long GRB supernova bumps, but fainter. All properties of GRB 040924 are consistent with an origin in the core-collapse of a massive star: the supernova, the spectrum and SED of the host and the afterglow.