The Jet Properties and Accretion Regime for the Blazar Sequence

Abstract: The puzzling bimodality displayed by blazars on the broad-band spectral indices ($α<em>{\rm ro}-α</em>{\rm ox}$) plane is still an open question. To investigate its physical origin, we compiled a sample comprising 136 flat-spectrum radio quasars (FSRQs), 64 low-synchrotron-peaked BL Lac objects (LBLs), 39 intermediate-synchrotron-peaked BL Lac objects (IBLs), and 105 high-synchrotron-peaked BL Lac objects (HBLs). Our results show that (FSRQs+LBLs)$\rightarrow$IBLs$\rightarrow$HBLs follow a \reflectbox{$\angle$}-shaped evolutionary sequence on the $α<em>{\rm ro}-α</em>{\rm ox}$ plane, with the average 5\,GHz radio Doppler factor, jet power, and accretion Eddington ratio decreasing along this \reflectbox{$\angle$}-shaped track. Additionally, the distributions of $α<em>{\rm ox}$ exhibit only slight differences among blazar subclasses, whereas those of $α</em>{\rm ro}$ and $α<em>{\rm rx}$ differ significantly. Our results can be naturally explained if the distribution of blazars on the $α</em>{\rm ro}-α<em>{\rm ox}$ plane is regulated by the accretion rates and the Doppler beaming effect across different bands ($δ_i<sup>{q_i}/δ_j<sup>{q_j}$). However, $δ_i/δ_j$ plays a pivotal role in the distribution, while jet shape ($q=2(3)+α</em>ν$) and spectral indices ($α_ν$) play a minor role. Our results suggest that FSRQs+LBLs are strong-jet sources coupled with radiatively efficient accretion, and their jets are accelerated to a larger distance from the core and still maintain relativistic speeds down to the radio region; HBLs are weak-jet sources coupled with radiatively inefficient accretion, and their jets may begin to decelerate in the optical region and exhibit sub-relativistic speeds as they propagate to the radio region. IBLs are regarded as transitional objects between FSRQs+LBLs and HBLs.