GRB 970228 and GRB 980329 and the Nature of Their Host Galaxie

Abstract

We find that the local galactic extinction towards the field of gamma-ray burst GRB970228 is $A_V=1.09^{+0.10}_{-0.20}$, which implies a substantial dimming and change in the spectral slope of the intrinsic GRB970228 afterglow. We measure a color $(V_{606}-I_{814})_{ST} = -0.18^{+0.51}_{-0.61}$ for the extended source coincident with the afterglow. Taking into account our measurement of the extinction toward this field, this color implies that the extended source is most likely a galaxy undergoing star formation, in agreement with our earlier conclusion (\cite{CL98}). In a separate analysis, we find that the inferred intrinsic spectrum of the GRB 980329 afterglow is consistent with the predictions of the simplest relativistic fireball model. We also find that the intrinsic spectrum of the afterglow is extincted both by dust (source frame $A_V \ga 1$ mag), and that the shape of the extinction curve is typical of young star-forming regions like the Orion Nebula but is not typical of older star-forming or starburst regions. The $\approx$ 2 mag drop between the $R$ and the $I$ bands can be explained by the far-ultraviolet non-linear component of the extinction curve if $3 \la z \la 4$, and by the 2175 \AA\ bump if $z \approx 2$; other redshifts are not consistent with the observational data, given our general model.