Testing a new view of Gamma-Ray Burst afterglows

The optical and X-ray light-curves of long Gamma Ray Bursts (GRBs) often show a complex evolution and in most cases do not track each other. This behaviour can not be easily explained by the simplest standard afterglow models. A possible interpretation is to consider the observed optical and X-ray light-curves as the sum of two separate components. This scenario requires the presence of a spectral break between these bands. One of the aims of this work is to test whether such a break is present within the observed Swift XRT energy range. We analyse the X-ray afterglow spectra of a sample of 33 long GRBs with known redshift, good optical photometry and published estimate of the host galaxy dust absorption A_V(host). We find that indeed in 7 bright events a broken power-law provides a fit to the data that is better than a single power-law model. For 8 events, instead, the X-ray spectrum is better fitted by a single power-law. We discuss the role of these breaks in connection to the relation between the host hydrogen column density N_H(host) and A_V(host) and check the consistency of the X-ray spectral breaks with the optical bands photometry. We analyse the optical to X-ray spectral energy distributions at different times and find again consistency with two components interpretation.