Does black hole spin play a key role in the FSRQ/BL Lac dichotomy?

Abstract

Blazars are characterized by large intensity and spectral v ariations across the electromagnetic spectrum It is believed that jets emerging from them are almo st aligned with the line-of-sight. The major- ity of identified extragalactic sources in γ -ray catalogs of EGRET and Fermi are blazars. Observational ly, blazars can be divided into two classes: flat spectrum radio q uasars (FSRQs) and BL Lacs. BL Lacs usually exhibit lower γ -ray luminosity and harder power law spectra at γ -ray energies than FSRQs. We attempt to explain the high energy properties of FSRQs and BL Lacs from F ermi γ -ray space telescope observations. It was argued previously that the difference in accretion rate s is mainly responsible for the large mismatch in observed luminosity in γ -ray. However, when intrinsic luminosities are derived by c orrecting for beaming effects, this difference in γ -ray luminosity between the two classes is significantly red uced. In order to ex- plain this difference in intrinsic luminosities, we propos e that spin plays an important role in the luminosity distribution dichotomy of BL Lacs and FSRQs. As the outflow po wer of a blazar increases with increasing spin of a central black hole, we suggest that the spin plays a c rucial role in making BL Lac sources low luminous and slow rotators compared to FSRQ sources.