The First Infrared Study of the Close Environment of a Long Gamma-Ray Burst

We present a characterization of the close environment of GRB 980425 based on 5-160 μm spectro-imaging obtained with Spitzer. The gamma-ray burst GRB 980425 occurred in a nearby (z = 0.0085) SBc-type dwarf galaxy at a projected distance of 900 pc from an H II region with strong signatures of Wolf-Rayet (WR) stars. While this "WR region" produces less than 5% of the B-band emission of the host, we find that it is responsible for 45% ± 10% of the total infrared luminosity, with a maximum contribution reaching 75% at 25-30 μm. This atypical property is rarely observed among morphologically relaxed dwarfs, suggesting a strong causal link with the gamma-ray burst (GRB) event. The luminosity of the WR region (L_(8-1000 μm) = 4.6 × 10^8 L_☉), the peak of its spectral energy distribution at ≾100 μm, and the presence of highly ionized emission lines (e.g., [Ne III]) also reveal extremely young (