We present a characterization of the close environment of GRB 980425 based on 5–160 \mu m 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 HIIÂ region with strong signatures of Wolf-Rayet ( WR ) stars .
While this “ WRÂ region ” produces less than 5 % of the B -band emission of the host , we find that it is responsible for 45 \pm 10 % of the total infrared luminosity , with a maximum contribution reaching 75 % at 25–30 \mu m. This atypical property is rarely observed among morphologically-relaxed dwarves , suggesting a strong causal link with the GRB event .
The luminosity of the WRÂ region ( L _ { 8 - 1000 \mu m } = 4.6 \times 10 ^ { 8 } L _ { \odot } ) , the peak of its spectral energy distribution at \lesssim 100 \mu m and the presence of highly-ionized emission lines ( e.g. , [ NeIII ] ) also reveal extremely young ( < 5 Myr ) star-forming activity , with a typical time-scale of only 47 Myr 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 _ { 9.7 \mu m } \sim 0.015 ) , this suggests that the WRÂ 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 .