2007MNRAS.378.1043J

We constrain the circumburst medium profile with the rise behaviour of the very early afterglow light curves of gamma-ray bursts (GRBs). Using this method, we find a constant and low-density medium profile for GRB 060418 and GRB 060607A, which is consistent with the inference from the late afterglow data. In addition, we show that the absence of the infrared flashes in these two afterglows is consistent with the standard hydrodynamical external reverse shock model. Although a highly magnetized model can explain the data, it is no longer demanded. A weak reverse shock in the standard hydrodynamical model is achievable if the typical synchrotron frequency is already below the band at the shock-crossing time.