Features of Structure and Absorption in the Jet-Launching Region of M87

Wei Zhao,Xiaopeng Cheng,Xiaoyu Hong,Tao An,Fang Wu,Xiaofeng Li

doi:10.3390/galaxies7040086

Abstract

M87 is one of the best available source for studying the AGN jet-launching region. To enrich our knowledge of this region, with quasi-simultaneous observations using VLBA at 22, 43 and 86 GHz, we capture the images of the radio jet in M87 on a scale within several thousand R s . Based on the images, we analyze the transverse jet structure and obtain the most accurate spectral-index maps of the jet in M87 so far, then for the first time, we compare the results of the two analyses and find a spatial association between the jet collimations and the local enhancement of the density of external medium in the jet-launching region. We also find the external medium is not uniform, and greatly contributes to the free-free absorption in this region. In addition, we find for the jet in M87, its temporal morphology in the launching region may be largely affected by the local, short-lived kink instability growing in itself.

Highlights

M87 is a nearby (D∼16.7 Mpc, [1]) FRI radio galaxy which harbors a central black hole with a mass reported from 2.4 to 7.2 × 109 Msun [2,3,4]
The structure of the jet in its launching region has been determined with Very Long Baseline Interferometry (VLBI) monitoring, that smears out the variability details by stacking multi-epoch observations at 22, 43 and 86 GHz [7,8,9]: the limb-brightened jet structure starts at a projected distance down to 7Rs to the super massive black hole (SMBH), with an apparent opening angle wider than 100◦ [9,10]; as the limb-brightened jet propagates outward, it has to experience multiple expansions followed with subsequent recollimations [8,9] i.e., “collimation regimes” and reaches an equilibrium parabolic expansion in several thousand Rs, and keeps the shape until 105 Rs [11,12]
300–2000Rs from the central engine, in the hot accretion flow models like ADIOS, the external medium interacting with an AGN jet on this scale is believed to be “winds”, i.e., the moderately magnetized, non-relativistic un-collimated, extremely hot and generally fully ionized gas outflows launched from the accretion disk

Summary

Introduction

M87 is a nearby (D∼16.7 Mpc, [1]) FRI radio galaxy which harbors a central black hole with a mass reported from 2.4 to 7.2 × 109 Msun [2,3,4]. M87 is a prototype to study the AGN jet-launching region as well as the vicinity of the SMBH. The structure of the jet in its launching region has been determined with VLBI monitoring, that smears out the variability details by stacking multi-epoch observations at 22, 43 and 86 GHz [7,8,9]: the limb-brightened jet structure starts at a projected distance down to 7Rs to the SMBH, with an apparent opening angle wider than 100◦ [9,10]; as the limb-brightened jet propagates outward, it has to experience multiple expansions followed with subsequent recollimations [8,9] i.e., “collimation regimes” and reaches an equilibrium parabolic expansion in several thousand Rs , and keeps the shape until 105 Rs [11,12]. Besides the motion along the jet, the jet flow rotates around the axis clockwisely, and the toroidal component of magnetic field

Results

Discussion

Conclusion