2009MNRAS.396L.105G

Flat-spectrum radio quasars (FSRQs) and BL Lac objects detected in the first three months of the Fermi survey neatly separate in the γ-ray spectral index versus γ-ray luminosity plane. BL Lac objects are less luminous and have harder spectra than broad-line blazars. We suggest that this division has its origin in the different accretion regimes of the two classes of objects. Using the γ-ray luminosity as a proxy for the observed bolometric one, we show that the boundary between the two subclasses of blazars can be associated with the threshold between the regimes of optically thick accretion discs and of radiatively inefficient accretion flows, which lies at an accretion rate of the order of 10^–2 the Eddington rate. The spectral separation in hard (BL Lacs) and soft (FSRQs) objects can then result from the different radiative cooling suffered by the relativistic electrons in jets propagating in different ambients. We argue that the bulk of the most luminous blazars already detected by Fermi should be characterized by large black hole masses, around 10⁹ solar masses, and predict that lowering the γ-ray flux threshold the region of the α_γ-L_γ plane corresponding to steep spectral indices and lower luminosities will be progressively populated by FSRQs with lower mass black holes, while the region of hard spectra and large luminosities will remain forbidden.