Spectral analysis of a parsec-scale jet in M 87: Observational constraint on the magnetic field strengths in the jet

Hyunwook Ro, Motoki Kino, Bong Won Sohn, Kazuhiro Hada, Jongho Park, Masanori Nakamura, Yuzhu Cui, Kunwoo Yi, Aeree Chung, Jeffrey Hodgson, Tomohisa Kawashima, Tao An, Sascha Trippe, Juan Carlos Algaba, Jae Young Kim, Satoko Sawada-Satoh, Kiyoaki Wajima, Zhiqiang Shen, Xiaopeng Cheng, Ilje ChoWu Jiang, Taehyun Jung, Jee Won Lee, Kotaro Niinuma, Junghwan Oh, Fumie Tazaki, Guang Yao Zhao, Kazunori Akiyama, Mareki Honma, Jeong Ae Lee, Rusen Lu, Yingkang Zhang, Keiichi Asada, Lang Cui, Yoshiaki Hagiwara, Tomoya Hirota, Noriyuki Kawaguchi, Shoko Koyama, Sang Sung Lee, Se Jin Oh, Koichiro Sugiyama, Mieko Takamura, Xuezheng Wang, Ju Yeon Hwang, Dong Kyu Jung, Hyo Ryoung Kim, Jeong Sook Kim, Hideyuki Kobayashi, Chung Sik Oh, Tomoaki Oyama, Duk Gyoo Roh, Jae Hwan Yeom

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Context. Because of its proximity and the large size of its black hole, M 87 is one of the best targets for studying the launching mechanism of active galactic nucleus jets. Currently, magnetic fields are considered to be an essential factor in the launching and accelerating of the jet. However, current observational estimates of the magnetic field strength of the M 87 jet are limited to the innermost part of the jet (≲100 rs) or to HST-1 (∼105 rs). No attempt has yet been made to measure the magnetic field strength in between. Aims. We aim to infer the magnetic field strength of the M 87 jet out to a distance of several thousand rs by tracking the distance-dependent changes in the synchrotron spectrum of the jet from high-resolution very long baseline interferometry observations. Methods. In order to obtain high-quality spectral index maps, quasi-simultaneous observations at 22 and 43 GHz were conducted using the KVN and VERA Array (KaVA) and the Very Long Baseline Array (VLBA). We compared the spectral index distributions obtained from the observations with a model and placed limits on the magnetic field strengths as a function of distance. Results. The overall spectral morphology is broadly consistent over the course of these observations. The observed synchrotron spectrum rapidly steepens from α22-43GHz ∼-0.7 at ∼2 mas to α22-43GHz ∼-2.5 at ∼6 mas. In the KaVA observations, the spectral index remains unchanged until ∼10 mas, but this trend is unclear in the VLBA observations. A spectral index model in which nonthermal electron injections inside the jet decrease with distance can adequately reproduce the observed trend. This suggests the magnetic field strength of the jet at a distance of 2-10 mas (∼900 rs-∼4500 rs in the deprojected distance) has a range of B = (0.3-1.0-G)(z/2mas)-0.73. Extrapolating to the Event Horizon Telescope scale yields consistent results, suggesting that the majority of the magnetic flux of the jet near the black hole is preserved out to ∼4500 rs without significant dissipation.

Original languageEnglish
Article numberA159
JournalAstronomy and Astrophysics
StatePublished - 1 May 2023


  • Galaxies: active
  • Galaxies: individual: M 87
  • Galaxies: jets
  • Radio continuum: galaxies
  • Relativistic processes
  • Techniques: interferometric


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