Colors and absolute magnitudes of the optical afterglow of GRB060218

Abstract

We investigate the photometric properties of the optical transient associated with GRB060218. We determine the k-corrected R band absolute magnitude of this optical afterglow (OA) in ( - To = 1.5 days in the rest frame to be -19.5, which places it at the fainter end of the statistical distribution of the OAs of the long GRBs which spans about 3 mag. We find the available color indices (starting in t - T 0 ≈ 2 days) to be inconsistent with those of the ensemble of 25 OAs which we analyzed previously, suggesting thus that the emission is caused by the associated supernova SN 2006aj. We find prominent color changes during the evolution of this SN and show that they can be partly fitted to those of SN 1998bw if SN 2006aj evolves twice as fast. We argue that not only the color indices of the SN at a given moment, but also the evolution of these indices appear to be important for its classification and photometric study.