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The Jet Properties and Accretion Regime for the Blazar Sequence

  • Qingchen Long
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Abstract

The puzzling bimodality displayed by blazars on the broad-band spectral indices (αro−αox) 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)→IBLs→HBLs follow a \reflectbox{angle}-shaped evolutionary sequence on the αro−αox 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 αox exhibit only slight differences among blazar subclasses, whereas those of αro and αrx differ significantly. Our results can be naturally explained if the distribution of blazars on the αro−αox plane is regulated by the accretion rates and the Doppler beaming effect across different bands (δiqi/δjqj). However, δi/δj plays a pivotal role in the distribution, while jet shape (q=2(3)+αν) and spectral indices (αν) 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.

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Cite

BibTeXRIS
BibTeX
@misc{long2026properties,
  title = {{The Jet Properties and Accretion Regime for the Blazar Sequence}},
  author = {Qingchen Long},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.03435},
  eprint = {2609.03435},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.03435},
}
RIS
TY  - UNPB
TI  - The Jet Properties and Accretion Regime for the Blazar Sequence
AU  - Qingchen Long
PY  - 2026
DA  - 2026/09/03
PB  - arXiv
DO  - 10.48550/arxiv.2609.03435
UR  - https://arxiv.org/abs/2609.03435
AB  - The puzzling bimodality displayed by blazars on the broad-band spectral indices (α_ro−α_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)→IBLs→HBLs follow a \reflectbox{angle}-shaped evolutionary sequence on the α_ro−α_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_ox exhibit only slight differences among blazar subclasses, whereas those of α_ro and α_rx differ significantly. Our results can be naturally explained if the distribution of blazars on the α_ro−α_ox plane is regulated by the accretion rates and the Doppler beaming effect across different bands (δ_i^q_i/δ_j^q_j). However, δ_i/δ_j plays a pivotal role in the distribution, while jet shape (q=2(3)+α_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.
ER  -