The Very Red Afterglow of GRB 000418: Further Evidence for Dust Extinction in a Gamma-Ray Burst Host Galaxy*

We report near-infrared and optical follow-up observations of the afterglow of the GRB 000418 start- ing 2.5 days after the occurrence of the burst and extending over nearly 7 weeks. GRB 000418 represents the second case for which the afterglow was initially identi—ed by observations in the near-infrared. During the —rst 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 the temporal evolution of the afterglow —attens with respect to a simple power-law decay. Attributing this to an underlying host galaxy, we —nd its magnitude to be R \ 23.9 and an intrinsic afterglow decay slope of 1.22. The afterglow was very red with R(K B 4 mag. The observations can be explained by an adiabatic, spherical —reball solution and a heavy reddening due to dust extinction in the host galaxy. This supports the picture that (long) bursts are associated with events in star-forming regions. Subject heading: gamma rays: bursts