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

Abstract

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 > 10⁴¹ ergs s^-1. The central black holes in these sources have masses of 10⁸-10¹⁰ M_☉, if the matter is accreting at the rate of 0.025_Edd. For the sources with L_Hβ < 10⁴¹ 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) × 10⁸ 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.