The extremely red QSO 1413 + 135 shows characteristics typical of BL Lac objects - no emission lines, is embedded in a luminous galaxy, is subject to rapid radio and IR variability, has an inverted radio spectrum and shows substantial IR polarization. The rapid steepening of the nonthermal spectrum at λ < 5 μm is interpreted as synchrotron emission from an electron distribution that ends sharply at some maximum energy. The X-ray emission is probably inverse Compton radiation. We calculate that the optically thin emitting region is compact, has a large magnetic field and exhibits bulk relativistic motion towards the observer. The most energetic electrons in the emitting region have a Lorentz factor of 102-103.
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