The extreme, red afterglow of GRB 060923A: Distance or dust?

TL;DR

This paper reports the discovery of a very red GRB afterglow, initially suggesting a high redshift, but later evidence indicates dust extinction at a moderate redshift, highlighting challenges in identifying distant GRBs.

Contribution

It presents the first near-infrared detection of GRB 060923A's afterglow and demonstrates how dust can mimic high-redshift signatures in GRB observations.

Findings

The afterglow was the faintest detected in K-band.

Optical and spectroscopic data suggest a moderate redshift (z<2.8).

Dust extinction likely causes the very red afterglow color.

Abstract

Gamma-ray bursts are powerful probes of the early universe, but locating and identifying very distant GRBs remains challenging. We report here the discovery of the K-band afterglow of Swift GRB 060923A, imaged within the first hour post-burst, and the faintest so far found. It was not detected in any bluer bands to deep limits, making it a candidate very high redshift burst (z>11). However, our later-time optical imaging and spectroscopy reveal a faint galaxy coincident with the GRB position which, if it is the host, implies a more moderate redshift (most likely z<2.8) and therefore that dust is the likely cause of the very red afterglow colour. This being the case, it is one of the few instances so far found of a GRB afterglow with high dust extinction.