Luminosity and colors of the early optical transient of GRB060218 in the context of the optical afterglows

摘要

We investigate the luminosity and colors of the optical afterglow (OA) of GRB060218 during t — To < 2 days in the context of the OAs of long GRBs. The R-band absolute magnitude —19.5 at (t -To)_(rest) = 1.5 d places this event below or at the lower end of the statistical distribution of the spectrally homogeneous ensemble of 23 OAs of long GRBs which we considered in our previous work [13, 14]. We find that the color indices of this phase are not consistent with those of the ensemble of OAs of GRBs, but their time evolution is. No variations of reddening even in the initial phase of these OAs are present. We argue that the optical and UV emission is not a simple continuation (Rayleigh-Jeans tail) of the X-ray emission.We also show that although both GRB060218 and XRF030723 possess a very soft gamma-ray spectrum, sorting them to X-ray flashes, the color indices of their early OAs are mutually discordant.