Evolution of the spectral indices from 1996 to 2010. Please note the different axis scales. The spectral index α is defined via S_ν ∝ ν^−α. Errorbars along the time axes denote the bin sizes, error bars along the α axes denote the statistical 1σ errors. Horizontal dashed lines (where visible) mark the α = 0 lines. For each source we chose the bins such that each bin covers a phase of similar flux levels (quiescent phases, flares).