The short GRB070707 afterglow and its very faint host galaxy

TL;DR

This study analyzes the afterglow of short GRB 070707, revealing a very faint host galaxy and a steep optical decay that challenges existing models, suggesting prolonged central engine activity or alternative explanations.

Contribution

It provides detailed multi-epoch observations of a short GRB afterglow and identifies the faintest host galaxy yet detected for such events, challenging previous assumptions.

Findings

Optical afterglow decayed steeply with index > 3

Host galaxy is extremely faint, with luminosity comparable to long GRBs

Steep decay challenges external shock models for short GRB afterglows

Abstract

We present the results from an ESO/VLT campaign aimed at studying the afterglow properties of the short/hard gamma ray burst GRB 070707. Observations were carried out at ten different epochs from ~0.5 to ~80 days after the event. The optical flux decayed steeply with a power-law decay index greater than 3, later levelling off at R~27.3 mag; this is likely the emission level of the host galaxy, the faintest yet detected for a short GRB. Spectroscopic observations did not reveal any line features/edges that could unambiguously pinpoint the GRB redshift, but set a limit z < 3.6. In the range of allowed redshifts, the host has a low luminosity, comparable to that of long-duration GRBs. The existence of such faint host galaxies suggests caution when associating short GRBs with bright, offset galaxies, where the true host might just be too dim for detection. The steepness of the decay of the optical afterglow of GRB 070707 challenges external shock models for the optical afterglow of short/hard GRBs. We argue that this behaviour might results from prolonged activity of the central engine or require alternative scenarios.