Spectral analysis of a parsec-scale jet in M87: Observational constraint on the magnetic field strengths in the jet
Creators: name: Ro, Hyunwook
name: Kino, Motoki
name: Sohn, Bong Won
name: Hada, Kazuhiro
name: Park, Jongho
name: Nakamura, Masanori
name: Cui, Yuzhu
name: Yi, Kunwoo
name: Chung, Aeree
name: Hodgson, Jeffrey
name: Kawashima, Tomohisa
name: An, Tao
name: Trippe, Sascha
name: Algaba, Juan-Carlos
name: Kim, Jae-Young
name: Sawada-Satoh, Satoko
name: Wajima, Kiyoaki
name: Shen, Zhiqiang
name: Cheng, Xiaopeng
name: Cho, Ilje
name: Jiang, Wu
name: Jung, Taehyun
name: Lee, Jee-Won
name: Niinuma, Kotaro
name: Oh, Junghwan
name: Tazaki, Fumie
name: Zhao, Guang-Yao
name: Akiyama, Kazunori
name: Honma, Mareki
name: Lee, Jeong Ae
name: Lu, Rusen
name: Zhang, Yingkang
name: Asada, Keiichi
name: Cui, Lang
name: Hagiwara, Yoshiaki
name: Hirota, Tomoya
name: Kawaguchi, Noriyuki
name: Koyama, Shoko
name: Lee, Sang-Sung
name: Oh, Se-Jin
name: Sugiyama, Koichiro
name: Takamura, Mieko
name: Wang, Xuezheng
name: Hwang, Ju-Yeon
name: Jung, Dong-Kyu
name: Kim, Hyo-Ryoung
name: Kim, Jeong-Sook
name: Kobayashi, Hideyuki
name: Oh, Chung-Sik
name: Oyama, Tomoaki
name: Roh, Duk-Gyoo
name: Yeom, Jae-Hwan
Publisher: name: arXiv
Year: 2023
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: Cosmology and Nongalactic Astrophysics (astro-ph.CO)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: Because of its proximity and the large size of its black hole, M87 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 M87 jet are limited to the innermost part of the jet or to HST-1. No attempt has yet been made to measure the magnetic field strength in between. We aim to infer the magnetic field strength of the M87 jet out to a distance of several thousand $r_s$ by tracking the distance-dependent changes in the synchrotron spectrum of the jet from high-resolution very long baseline interferometry observations. 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 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. The overall spectral morphology is broadly consistent over the course of these observations. The observed synchrotron spectrum rapidly steepens from $α_{22-43 GHz}$ ~ -0.7 at ~ 2 mas to $α_{22-43 GHz}$ ~ -2.5 at ~ 6 mas. 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 $r_s$ - ~ 4500 $r_s$ in the deprojected distance) has a range of $B=(0.3 - 1.0 G)(z/2 mas)^{-0.73}$. Extrapolating to the EHT scale yields consistent results, suggesting that the majority of the magnetic flux of the jet near the black hole is preserved out to ~ 4500 $r_s$ without significant dissipation.
description: 16 pages, 9 figures. Accepted for publication in Astronomy &amp; Astrophysics