The first Infrared study of the close environment of a long Gamma-Ray
  Burst
  We present a characterization of the close environment of GRB980425 based on
5-160mic spectro-imaging obtained with Spitzer. The Gamma-Ray Burst GRB980425
occurred in a nearby (z=0.0085) SBc-type dwarf galaxy, at a projected distance
of 900pc from an HII 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 25-30mic. This atypical
property is rarely observed among morphologically-relaxed dwarves, suggesting a
strong causal link with the GRB event. The luminosity of the WR region
(L_8-1000mic=4.6x10^8 Lsol), the peak of its spectral energy distribution at
<~100mic and the presence of highly-ionized emission lines (e.g., [NeIII]) also
reveal extremely young (<5Myr) star-forming activity, with a typical time-scale
of only 47Myr to double the stellar mass already built. Finally, the mid-IR
over B-band luminosity ratio in this region is substantially higher than in
star-forming galaxies with similar L_IR, but it is lower than in young
dust-enshrouded stellar clusters. Considering the modest obscuration measured
from the silicate features (tau_9.7mic ~ 0.015), this suggests that the WR
region is dominated by one or several star clusters that have either partly
escaped or cleared out their parent molecular cloud. Combined with the
properties characterizing the whole population of GRB hosts, our results
reinforce the idea that long GRBs mostly happen within or in the vicinity of
relatively unobscured galactic regions harboring very recent star formation.
