Innermost collimation structure of the M87 jet down to ~ten Schwarzschild radii
We investigated the detailed inner jet structure of M87 using the 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 a size of the core region transverse to the jet axis, defined as $W_{\rm c}$, at each frequency ν, and found a relation between $W_{\rm c}$ and $ν$ as $W_{\rm c}(ν) \propto ν^{-0.71\pm0.05}$. Then, by combining $W_{\rm c}(ν)$ and the frequency dependence of the core position $r_{\rm c}(ν)$, which was obtained by our previous study, we have constructed a collimation profile of the innermost jet $W_{\rm c}(r)$ down to ~10 Schwarzschild radii ($R_{\rm s}$) from the central black hole. We found that $W_{\rm c}(r)$ 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 $R_{\rm s}$. Closer to the black hole, the measured radial profile suggests a possible change of the jet collimation shape from the outer parabolic one, in which the jet shape tends to become more radially-oriented. This could be related to a magnetic collimation process or/and interaction with surrounding materials at the jet base. The present results shed light on the importance of higher-sensitivity/resolution imaging studies for M87 at 86, 43 and also 22 GHz, and should be examined more rigorously.