The high efficiency jets magnetically accelerated from a thin disk in powerful lobe-dominated FRII radio galaxies

Abstract

A maximum jet efficiency line $R\sim 25$ ($R=L_{\rm jet}/L_{\rm bol}$), found in FRII radio galaxies by Fernandes et al., was extended to cover the full range of jet power by Punsly. Recent general relativistic magnetohydrodynamic (GRMHD) simulations on jet formation mainly focused on the enhancement of jet power. In this work, we suggest that the jet efficiency could be very high even for conventional jet power if the radiative efficiency of disk were much smaller. We adopt the model of a thin disk with magnetically driven winds to investigate the observational high efficiency jets in FRII radio galaxies. It is found that the structure of a thin disk can be significantly altered by the feedback of winds. The temperature of disk gradually decreases with increasing magnetic field; the disk density, surface density and pressure also change enormously. The lower temperature and higher surface density in inner disk result in the rapid decrease of radiative efficiency. Thus, the jet efficiency is greatly improved even the jet power is conventional. Our results can explain the observations quite well. A theoretical maximum jet efficiency $R \sim 1000$ suggested by our calculations is large enough to explain all the high jet efficiency in observations even considering the episodic activity of jets.