2013ApJ...775...70H

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_cand ν: W_c(ν)∝ν^{–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.