Energy densities of magnetic field and relativistic electrons at the innermost region of the M87 jet

M Kino,K Hada,F Takahara,A Doi,José L Gómez

doi:10.1051/epjconf/20136101009

Abstract

We explore energy densities of magnetic fields and relativistic electrons in M87 jet. Since the radio core at the base of the M87 jet is the optically thick surface against synchrotron self absorption (SSA), observations directly give the size and turnover frequency for SSA. Using the observed angular diameter 0.11 mas, which corresponds to 16 Schwarzschild radii of the central black hole with 6 × 109 solar mass, and the flux density of the radio core at 43 GHz, we estimate the energy densities of magnetic field (UB) and relativistic electrons (Ue) by comparing the standard SSA formula to the observed radio core. Together with the allowed total kinetic power of the M87 jet, we find that (i) the allowed B is limited in the range 2 G ≤ B ≤ 13 G, and that (ii) 0:18 ≤ Ue/UB ≤ 66 holds. Our results significantly constrain formation mechanism of relativistic jets in active galactic nuclei.

Highlights

Formation mechanism of relativistic jets in active galactic nuclei (AGNs) remains as a longstanding unresolved problem in astrophysics
An importance of estimations of magnetic field energy density (UB) and electron one (Ue) for resolving the formation mechanism had been emphasized (e.g., [13]; [5]), it is not observationally clear whether UB or Ue is dominant at a jet base
Since the radio core surface corresponds to the optically-thick surface at each frequency, a synchrotron-self-absorption (SSA) turnover frequency νssa is identical to the observed frequency itself

Summary

Introduction

Formation mechanism of relativistic jets in active galactic nuclei (AGNs) remains as a longstanding unresolved problem in astrophysics. An importance of estimations of magnetic field energy density (UB) and electron one (Ue) for resolving the formation mechanism had been emphasized (e.g., [13]; [5]), it is not observationally clear whether UB or Ue is dominant at a jet base. No one has succeeded in firmly obtaining a robust estimation of an actual value of Ue/UB at a jet base. (1) H11 succeed in directly measuring core-shift phenomena at the jet base of M87 at 2, 5, 8, 15, 22 and 43 GHz. The radio core position at each frequency has been obtained by the astrometric observation of the core shift along the M87 jet. To measure the width of the core, a single, full-width-half-maximum (FWHM) Gaussian is fitted for observed core in the perpendicular direction to the jet axis and we derive the width of the core (θFWHM). Using θFWHM at 43GHz, we can estimate model-independent value of Ue/UB in the 43GHz core of M87 for the first time

One-zone Model

Gpc 1 mas

Summary and discussions