Anomalous X-ray emission in GRB 060904B: A nickel line?

Context.The detection of an extra component in the GRB 060904B X-ray spectra in addition to the standard single power-law behaviour has recently been reported in the literature. This component can be fit with different models; the addition of a spectral line provides the best correspondence.Aims.We investigate the physical properties that the surrounding medium must have in order to produce a spectral feature that can explain the detected emission.Methods.We analyse and discuss how and if the detected spectral excess fits in different theoretical models developed to explain the nature of line emission during the afterglow phase of Gamma-Ray Bursts (GRBs). Transmission and reflection models have been considered.Results.Given the high value () of the Thomson optical depth, the emission is likely to arise in a reflection scenario. Within reflection models, the external reflection geometry fails to predict the observed luminosity. On the contrary, the detected feature can be explained in a funnel scenario with typical opening angle , nickel mass ∼ and . For , assuming the reprocessing material to be in the SN shell, the detected emission implies a nickel mass ∼ at and a metallicity ∼ times the solar value. If the giant X-ray flare that dominates the early XRT light curve is identified as the ionizing source, the SN expansion began ∼ before the GRB event.