KaVA (KVN and VERA Array) is a new combined VLBI array with KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). First, we briefly review the imaging capabilities of KaVA array which actually achieves more than three times bette
r dynamic range than that achieved by VERA alone. The KaVA images clearly show detailed structures of extended radio jets in AGNs. Next, we represent the key science program to be led by KaVA AGN sub working group. We will conduct the monitoring observations of Sgr A* and M87 because of the largeness of their central super-massive black hole angular sizes. The main science goals of the program are (i) testing magnetically-driven-jet paradigm by mapping velocity fields of the M87 jet, and (ii) obtaining tight constraints on physical properties of radio emitting region in Sgr A*.
We explore energy densities of magnetic field 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 as 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 field ($U_{B}$) and relativistic electrons ($U_{e}$) based on the standard SSA formula. Imposing the condition that the Poynting power and relativistic electron one should be smaller than the total power of the jet, we find that (i) the allowed range of the magnetic field strength ($B_{tot}$) is from 1 G to 15 G, and that (ii) $1 times 10^{-5} < U_{e}/U_{B} < 6 times 10^{2}$ holds. The uncertainty of $U_{e}/U_{B}$ comes from the strong dependence on the angular size of the radio core and the minimum Lorentz factor of non-thermal electrons ($gamma_{e,min}$) in the core. It is still open that the resultant energetics is consistent with either the magnetohydrodynamic jet or with kinetic power dominated jet even on ~10 Schwarzschild radii scale.
