The luminous host galaxy, faint supernova and rapid afterglow rebrightening of GRB 100418A

TL;DR

This paper studies GRB 100418A's afterglow, host galaxy, and associated faint supernova, revealing unusual rebrightening behavior and a diverse range of supernova luminosities, expanding understanding of GRB environments and progenitors.

Contribution

It provides detailed multi-wavelength observations of GRB 100418A, highlighting its atypical optical rebrightening and faint supernova, and offers insights into the host galaxy's properties and interstellar medium.

Findings

Fast optical rebrightening cannot be explained by standard models.

Detected a faint supernova with M_V = -18.5 mag, among the faintest for GRB-SNe.

Host galaxy shows two components with similar properties but different velocities.

Abstract

Long gamma-ray bursts give us the chance to study both their extreme physics and the star-forming galaxies in which they form. GRB 100418A, at a z = 0.6239, had a bright optical and radio afterglow, and a luminous star-forming host galaxy. This allowed us to study the radiation of the explosion as well as the interstellar medium of the host both in absorption and emission. We collected photometric data from radio to X-ray wavelengths to study the evolution of the afterglow and the contribution of a possible supernova and three X-shooter spectra obtained during the first 60 hr. The light curve shows a very fast optical rebrightening, with an amplitude of 3 magnitudes, starting 2.4 hr after the GRB onset. This cannot be explained by a standard external shock model and requires other contributions, such as late central-engine activity. Two weeks after the burst we detect an excess in the light curve consistent with a SN with peak absolute magnitude M_V = -18.5 mag, among the faintest GRB-SNe detected to date. The host galaxy shows two components in emission, with velocities differing by 130 km s^-1, but otherwise having similar properties. While some absorption and emission components coincide, the absorbing gas spans much higher velocities, indicating the presence of gas beyond the star-forming regions. The host has a star-formation rate of 12.2 M_sol yr^-1, a metallicity of 12 + log(O/H) = 8.55 and a mass of 1.6x10^9 M_sol. GRB 100418A is a member of a class of afterglow light curves which show a steep rebrightening in the optical during the first day, which cannot be explained by traditional models. Its very faint associated SN shows that GRB-SNe can have a larger dispersion in luminosities than previously seen. Furthermore, we have obtained a complete view of the host of GRB 100418A owing to its spectrum, which contains a remarkable number of both emission and absorption lines.