Evidence for the Evolutionary Sequence of Blazars: Different Types of Accretion Flows in BL Lacertae Objects

The limits on the mass of the black hole in 23 BL Lac objects are obtained from their luminosities of the broad emission line Hβ on the assumption that broad emission lines are emitted from clouds ionized by the radiation of the accretion disk surrounding a black hole. The distribution of line luminosity L Hβ of all these BL Lac objects suggests a bimodal nature, although this cannot be statistically proven on the basis of the present, rather small sample. We found that standard thin disks are probably in the sources with L Hβ > 10 41 ergs s -1 . The central black holes in these sources have masses of 10 8 -10 10 M ☉ , if the matter is accreting at the rate of 0.025 Edd . For the sources with L Hβ < 10 41 ergs s -1 , the accretion flows have transited from standard thin-disk type to the advection-dominated accretion flow type. The lower limits on the mass of the black hole in these sources are in the range of (1.66-24.5) × 10 8 M ☉ . The results support the evolutionary sequence of blazars from flat-spectrum radio quasar to low-energy-peaked BL Lac object to high-energy-peaked BL Lac object.