Evolution of the color indices in SN 2006aj associated with GRB 060218

¹ Astronomical Institute, Academy of Sciences of the Czech Republic, 251 65 Ondřejov, Czech Republic
e-mail: simon@asu.cas.cz
² INAF/IASF Bologna, via Gobetti 101, 40129 Bologna, Italy

Received: 19 March 2010
Accepted: 22 July 2010

Abstract

Aims. We analyze the photometric properties of SN 2006aj associated with GRB 060218. We also investigate this event in relation to other supernovae.

Methods. We determined the color indices from multiband photometry and investigated their time evolution by taking their time stretch factors into account. We used both the ground-based and UVOT/Swift data to ensure a coverage from the cosmic UV region (UVW1 (2510 Å), UVM2 (2170 Å), UVW2 (1880 Å)) to the I band.

Results. We find a large difference between the values and time evolution of the color indices of the early phase (t − T₀ ≤ 2.5 d) of the optical afterglow (OA) and the subsequent SN 2006aj. The clustering and small evolution of most color indices in UV in t − T₀ ≤ 2.5 d resemble the behavior seen in the ensemble of OAs of long GRBs. In our interpretation, the light of the early OA comes from far above the photosphere of the progenitor and its wind. The outer layers of SN 2006aj underwent the strongest evolution among the considered supernovae of various types. We find that the adjustment of time evolution of various color indices by stretching is very helpful in searching for the proper supernova type. We attribute this evolution of SN 2006aj to the exceptionally low mass of its ejecta and the structural changes of the region producing the GRB. In our interpretation, we observed the different regions simultaneously in the complicated, asymmetric shape of SN 2006aj. The colors in the UV band can be explained if line blanketing undergoes only small variations and the UV emitting area tends to shrink during the evolution of SN 2006aj over the time interval investigated. Our approach is also of general importance for investigating supernovae in OAs.