Figure 2.
The optical spectrum of the BL Lac object S2 0109+22 obtained with OSIRIS@GTC. Upper panel: flux calibrated spectrum well described by a power law with a spectral index α = −0.56 ± 0.06. Telluric absorption O2 and H20 bands are marked by ⊕. The only visible interstellar absorption line is Na I (5892 Å). Bottom panel: normalized spectrum.
