2015ApJ...803...30K

We explore the degree of magnetization at the jet base of M87 by using the observational data of the event horizon telescope (EHT) at 230 GHz obtained by Doeleman et al. By utilizing the method in Kino et al., we derive the energy densities of the magnetic fields (U_B) and electrons and positrons (U_±) in the compact region detected by EHT (the EHT region) with its FWHM size 40µas. First, we assume that an optically thick region for synchrotron self-absorption (SSA) exists in the EHT region. Then, we find that the SSA-thick region should not be too large, in order to not overproduce the Poynting power at the EHT region. The allowed ranges of the angular size and the magnetic-field strength of the SSA-thick region are 21 µas ≤ Θ_thick ≤ 26.3 µas and 50 G ≤ B_tot ≤ 124 G, respectively. Correspondingly, U_B ≫ U_± is realized in this case. We further examine the composition of plasma and energy density of protons by utilizing the Faraday rotation measurement at 230 GHz obtained by Kuo et al. Then, we find that U_B ≫ U_± + U_p still holds in the SSA-thick region. Second, we examine the case when the EHT region is fully SSA-thin. Then, we find that U_B ≫ U_± still holds unless protons are relativistic. Thus, we conclude that the magnetically driven jet scenario in M87 is viable in terms of energetics close to the Innermost Stable Circular Orbit scale unless the EHT region is fully SSA-thin and relativistic protons dominated.