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 A. Hodgson, Tomohisa Kawashima, Tao An, Sascha Trippe, Juan Carlos Algaba, Jae-Young Kim, Satoko Sawada‐Satoh, Kiyoaki Wajima, Zhi-Qiang Shen, Xiaopeng Cheng, Ilje Cho, Wu Jiang, Taehyun Jung, Jee Won Lee, Kotaro Niinuma, Junghwan Oh, Fumie Tazaki, Guangyao Zhao, Kazunori Akiyama, Mareki Honma, Jeong Ae Lee, Ru-Sen 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, Chungsik Oh, Tomoaki Oyama, Duk-Gyoo Roh, Jae-Hwan Yeom
Abstract
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 r s ) or to HST-1 (∼10 5 r s ). 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 r s 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 α 22 − 43 GHz ∼ −0.7 at ∼2 mas to α 22 − 43 GHz ∼ −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 r s − ∼4500 r s 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 r s without significant dissipation.