The very red afterglow of GRB 000418: Further evidence for dust extinctionin a gamma-ray burst host galaxy

We report near-infrared and optical follow-up observations of the afterglow of the GRB 000418 starting 2.5 days after the occurrence of the burst and extending over nearly 7 weeks. GRB 000418 represents the second case for wh ich the afterglow was initially identified by observations in the near-infr ared. During the first 10 days its R-band afterglow was well characterized by a single power-law decay with a slope of 0.86. However, at later times t he temporal evolution of the afterglow flattens with respect to a simple po wer-law decay. Attributing this to an underlying host galaxy, we find its m agnitude to be R = 23.9 and an intrinsic afterglow decay slope of 1.22. The afterglow was very red with R-K approximate to 4 mag. The observations can be explained by an adiabatic, spherical fireball solution and a heavy redd ening due to dust extinction in the host galaxy. This supports the picture that (long) bursts are associated with events in star-forming regions.