Univ Tubingen, Inst Astron & Astrophys, Abt Astron, D-72076 Tubingen, Germany
; Chinese Acad Sci, Inst High Energy Phys, High Energy Astrophys Lab, Beijing 100039, Peoples R China
; Observ Carnegie Inst Washington, Pasadena, CA 91101 USA
We investigate the relationship between accretion rates and the spectral energy distributions (SEDs) of BL Lac objects using a sample of objects for which published information on the host galaxies, emission-line luminosities, and peak frequencies and luminosities of their SEDs are available. The sample is composed of 43 BL Lac objects that have a relatively continuous distribution of peak frequencies. Under the assumption that the observed emission lines are photoionized by the central accretion disk, we use the line luminosities to estimate the accretion luminosities and hence accretion rates. We find that low-frequency peaked BL Lac objects (LBLs) span a wide range of accretion rates, whereas high-frequency peaked BL Lac objects (HBLs) cover a more restricted range of lower values. There appears to be a continuous distribution of accretion rates between the two subclasses of BL Lac objects. We find that the peak frequency of the SED, nu(pk), correlates with the accretion rate, approximately with the form nu(pk) proportional to Lambda(-3) in HBLs and nu(pk) proportional to Lambda(-0.25) in LBLs, where Lambdaequivalent toL(lines)/c(2). The peak luminosity of the SED is also correlated with. These results suggest that the accretion rate influences the shape of the SED in BL Lac objects. They also support models that couple the jet and the accretion disk. We present a physical scenario to account for the empirical trends.