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Peer-reviewedCC BY 4.0The Astrophysical Journal

The Relation between Magnetic-field Intensity and Size of the Emission Region in the High-synchrotron-peaked BL Lacertae Object Mrk 421

  • Y. B. Xu1,2
  • Y. G. Zheng2
  • L. Zhang1
  1. 1Yunnan University
  2. 2Yunnan Normal University
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Abstract

Magnetic effects are thought to dominate the formation of BL Lac object jets, but the role and influence of large-scale magnetic fields within the jets remains incompletely understood. We utilize a one-zone synchrotron-self-Compton (SSC) model with broken-power-law electron injection to reproduce multiwavelength spectral energy distributions (SEDs) of the high-synchrotron-peaked BL Lac object Mrk 421 for 73 activity states, constraining six key physical parameters. We investigate the correlation between magnetic-field strength B and emission-region size R and estimate the powers of relativistic electrons, cold protons, magnetic fields, and total jet power. We find a significant anticorrelation log B = ( − 0.93 ± 0.05 ) log R + ( 14.22 ± 0.83 ) . The derived slope close to −1 is compatible with theoretical expectations for a toroidal magnetic geometry, though parameter couplings inherent to the leptonic model cannot be fully excluded. Markov Chain Monte Carlo (MCMC) posterior analyses demonstrate that this B – R scaling is not merely an artefact of intra-epoch parameter degeneracy. On emission-region scales the jet is particle-kinetic-energy dominated; distinct electron-acceleration mechanisms (shocks, magnetic reconnection) operate across different activity states, and the conventional Poynting-flux-dominated scenario cannot account for the observed radiative power.

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Cite

BibTeXRIS
BibTeX
@article{xu2026relation,
  title = {{The Relation between Magnetic-field Intensity and Size of the Emission Region in the High-synchrotron-peaked BL Lacertae Object Mrk 421}},
  author = {Y. B. Xu and Y. G. Zheng and L. Zhang},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = nov,
  doi = {10.3847/1538-4357/aea615},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea615},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - The Relation between Magnetic-field Intensity and Size of the Emission Region in the High-synchrotron-peaked BL Lacertae Object Mrk 421
AU  - Y. B. Xu
AU  - Y. G. Zheng
AU  - L. Zhang
PY  - 2026
DA  - 2026/11/06
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/aea615
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/aea615
AB  - Magnetic effects are thought to dominate the formation of BL Lac object jets, but the role and influence of large-scale magnetic fields within the jets remains incompletely understood. We utilize a one-zone synchrotron-self-Compton (SSC) model with broken-power-law electron injection to reproduce multiwavelength spectral energy distributions (SEDs) of the high-synchrotron-peaked BL Lac object Mrk 421 for 73 activity states, constraining six key physical parameters. We investigate the correlation between magnetic-field strength B and emission-region size R and estimate the powers of relativistic electrons, cold protons, magnetic fields, and total jet power. We find a significant anticorrelation log B = ( − 0.93 ± 0.05 ) log R + ( 14.22 ± 0.83 ) . The derived slope close to −1 is compatible with theoretical expectations for a toroidal magnetic geometry, though parameter couplings inherent to the leptonic model cannot be fully excluded. Markov Chain Monte Carlo (MCMC) posterior analyses demonstrate that this B – R scaling is not merely an artefact of intra-epoch parameter degeneracy. On emission-region scales the jet is particle-kinetic-energy dominated; distinct electron-acceleration mechanisms (shocks, magnetic reconnection) operate across different activity states, and the conventional Poynting-flux-dominated scenario cannot account for the observed radiative power.
SN  - 1538-4357
SN  - 0004-637X
ER  -