Different Types of Accretion Flows Around Massive Black Holes in BL Lac Objects

The distribution of the line luminosity LHβ of 23 BL Lac objects suggests a bimodal nature.We found that standard thin disks probably reside in the sources with LHβ>10⁴¹ erg s^-1.For the sources with LHβ<10 ⁴¹ erg s^-1, the accretion flows have transited from the standard thin disk type to the ADAF type.We propose a new method to estimate the masses of the central black holes in BL Lac objects.