# Spectral analysis of a parsec-scale jet in M87: Observational constraint on the magnetic field strengths in the jet

**Authors:** 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 Cho  
**arXiv:** [2303.01014](https://arxiv.org/abs/2303.01014)  
**Date:** 2023-03-02  
**Summary page:** https://www.summarizepaper.com/en/paper/2303.01014v1/

## Key points

- M87 is a nearby galaxy with one of the largest known black holes
- Ideal for studying active galactic nucleus jets launched and accelerated by magnetic fields
- Researchers conducted high-resolution observations using KVN, VERA Array, and VLBA
- Nonthermal electron injections inside the jet decrease with distance causing observed synchrotron spectra to rapidly steepen
- Magnetic field strength in the M87 jet at distances between 2 - 10 mas has a range of $B=(0.3 - 1.0 G)(z/2 mas)^{-0.73}$
- Most of the magnetic flux near M87's black hole is preserved out to ~4500 $r_s$ without significant dissipation
- Findings provide new insights into the magnetic field strength of the M87 jet and shed light on its launching mechanism

## Summary

M87 is a nearby galaxy that hosts one of the largest black holes known to exist. Its proximity and the size of its black hole make it an ideal target for studying active galactic nucleus jets, which are thought to be launched and accelerated by magnetic fields. To gain insight into the magnetic field strength of the M87 jet, a team of researchers conducted high-resolution very long baseline interferometry observations at 22 and 43 GHz using the KVN and VERA Array (KaVA) and the VLBA. The researchers compared spectral index distributions obtained from their observations with a model and placed limits on magnetic field strengths as a function of distance. Their analysis revealed that nonthermal electron injections inside the jet decrease with distance, causing observed synchrotron spectra to rapidly steepen from $\alpha_{22-43 GHz}$ ~ -0.7 at ~2 mas to $\alpha_{22-43 GHz}$ ~ -2.5 at ~6 mas. This suggests that magnetic field strength in the M87 jet at distances between 2 - 10 mas (~900 $r_s$ - ~4500 $r_s$ in deprojected distance) has a range of $B=(0.3 - 1.0 G)(z/2 mas)^{-0.73}$. Extrapolating these results to larger scales yielded consistent results, suggesting that most of the magnetic flux near M87's black hole is preserved out to ~4500 $r_s$ without significant dissipation. These findings provide new insights into the magnetic field strength of the M87 jet and shed light on its launching mechanism; however, further observations will be needed to determine how this changes over time.