Colors of Optical Afterglows of GRBs and Their Time Evolution

摘要

The study of the color indices of 23 optical afterglows (OAs) of GRBs shows that the color variations during the decline of the OAs are very small during t – T0 < 10 days. The colors in the observer frame concentrate at (B – V)0 = 0.45 ± 0.15, (V – R)0 = 0.41 ± 0.09, (R – I)0 = 0.46 ± 0.13, (I – J)0 = 0.88 ± 0.15, but are more scattered in (U – B)0. These findings entirely confirm our previous ones, determined from a smaller ensemble of the OAs with a smaller range of the redshifts z. The strong concentration of the color indices of most OAs also suggests that the intrinsic reddening (i.e. inside their host galaxies) must be quite similar and relatively small for these events. The maximum reddening which can be allowed by the scatter of the colors of the individual OAs is about EB–V = 0.2. This suggests that they are not deep inside the star-forming regions or, alternatively, that the dust is destroyed by the energetic initial flash.