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 \nu, and found a relation between $W_{\rm c}$
and $\nu$ as $W_{\rm c}(\nu) \propto \nu^{-0.71\pm0.05}$. Then, by combining
$W_{\rm c}(\nu)$ and the frequency dependence of the core position $r_{\rm
c}(\nu)$, 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.
