The Innermost Collimation Structure of the M87 Jet Down to ~10 Schwarzschild Radii

We investigated the detailed inner jet structure of M87 using Very Long Baseline Array data at 2, 5, 8.4, 15, 23.8, 43, and 86 GHz, especially focusing on the multi-frequency properties of the radio core at the jet base. First, we measured the size of the core region transverse to the jet axis, defined as W _c, at each frequency ν, and found a relation between W _c and ν: W _c(ν)vpropν^{-0.71 ± 0.05}. Then, by combining W _c(ν) and the frequency dependence of the core position r _c(ν), which was obtained in our previous study, we constructed a collimation profile of the innermost jet W _c(r) down to ~10 Schwarzschild radii (R _s) from the central black hole. We found that W _c(r) smoothly connects with the width profile of the outer edge-brightened, parabolic jet and then follows a similar radial dependence down to several tens of R _s. Closer to the black hole, the measured radial profile suggests a possible change in the jet collimation shape from the outer parabolic one, where the jet shape tends to become more radially oriented. This result could be related to a magnetic collimation process or/and interactions with surrounding materials at the jet base. The present results shed light on the importance of higher-sensitivity/resolution imaging studies of M87 at 86, 43, and 22 GHz these studies should be examined more rigorously.