We explore energy densities of the magnetic fields and relativistic electrons in the M87 jet. Since the radio core at the jet base is identical to the optically thick surface against synchrotron self-absorption (SSA), the observing frequency is identical to the SSA turnover frequency. As a first step, we assume the radio core has a simple uniform sphere geometry. Using the observed angular size of the radio core measured by the Very Long Baseline Array at 43?GHz, we estimate the energy densities of magnetic fields (UB ) and relativistic electrons (Ue ) on the basis of the standard SSA formula. Imposing the condition that the Poynting power and kinetic power of relativistic electrons should be smaller than the total power of the jet, we find that (1) the allowed range of the magnetic field strength (B tot) is 1 G ≤ B tot ≤ 15 G and that (2) 1 × 10–5 ≤ Ue /UB ≤ 6 × 102 holds. The uncertainty of Ue /UB comes from the strong dependence on the angular size of the radio core and the minimum Lorentz factor of non-thermal electrons (γ e, min) in the core. It is still unsettled whether resultant energetics are consistent with either the magnetohydrodynamic jet or the kinetic power dominated jet even on the ~10 Schwarzschild radii scale.
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