Are jet speeds governed by accretion modes?
Abstract
Relativistic jets are observed in both stellar-mass black hole X-ray binaries (BHXRBs) and active galactic nuclei (AGNs), yet their bulk Lorentz factors differ systematically---those in black hole X-ray binaries are typically below ∼2, whereas AGN jets can reach ∼50. The origin of this discrepancy remains unclear. Searching the literature, we compile a sample of 333 AGNs with well-measured jet component motions, consisting of 270 quasars, 47 BL Lac objects, 10 FRI, and 6 FRII galaxies. We find that quasars/FRIIs exhibit minimal bulk Lorentz factors ranging from ∼1.0 to 41.5, with a mean of 11.5 (median 9.5). In contrast, BL Lac objects/FRIs show Γjet∼1.0--21.9, averaging 4.2 (median 1.5). These values, particularly the median, closely resemble those of BHXRBs, implying a strong correlation between jet speed and accretion mode. The Lorentz factor of a magnetically driven jet is mainly determined by the ratio of the magnetic pressure to rest mass energy density at the jet base. In BL Lacs/FRIs/BHXRBs, the field is maintained by the advection-dominated accretion flow (ADAF), and the gas at the ADAF surface is magnetically driven into the jets. In quasars/FRIIs, the field is maintained by the disc, while the jet base is connected to the corona. Our model calculations show that the disc field is always much stronger than that of the ADAF, and therefore leads to a larger Γjet, which can explain the systematic difference in Γjet between these two types of sources.
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BibTeX
@misc{tu2026speeds,
title = {{Are jet speeds governed by accretion modes?}},
author = {Hong Tu and Xinwu Cao and Andrzej A. Zdziarski},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.02550},
eprint = {2609.02550},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.02550},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Are jet speeds governed by accretion modes?
AU - Hong Tu
AU - Xinwu Cao
AU - Andrzej A. Zdziarski
PY - 2026
DA - 2026/09/03
PB - arXiv
DO - 10.48550/arxiv.2609.02550
UR - https://arxiv.org/abs/2609.02550
AB - Relativistic jets are observed in both stellar-mass black hole X-ray binaries (BHXRBs) and active galactic nuclei (AGNs), yet their bulk Lorentz factors differ systematically---those in black hole X-ray binaries are typically below ∼2, whereas AGN jets can reach ∼50. The origin of this discrepancy remains unclear. Searching the literature, we compile a sample of 333 AGNs with well-measured jet component motions, consisting of 270 quasars, 47 BL Lac objects, 10 FRI, and 6 FRII galaxies. We find that quasars/FRIIs exhibit minimal bulk Lorentz factors ranging from ∼1.0 to 41.5, with a mean of 11.5 (median 9.5). In contrast, BL Lac objects/FRIs show Γ_jet∼1.0--21.9, averaging 4.2 (median 1.5). These values, particularly the median, closely resemble those of BHXRBs, implying a strong correlation between jet speed and accretion mode. The Lorentz factor of a magnetically driven jet is mainly determined by the ratio of the magnetic pressure to rest mass energy density at the jet base. In BL Lacs/FRIs/BHXRBs, the field is maintained by the advection-dominated accretion flow (ADAF), and the gas at the ADAF surface is magnetically driven into the jets. In quasars/FRIIs, the field is maintained by the disc, while the jet base is connected to the corona. Our model calculations show that the disc field is always much stronger than that of the ADAF, and therefore leads to a larger Γ_jet, which can explain the systematic difference in Γ_jet between these two types of sources.
ER -