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 ( [ Castander & Lamb 1998 ] ) .
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 Å bump if z \approx 2 ; other redshifts are not consistent with the observational data , given our general model .