Determination of Central Engine Position and Accretion Disk Structure in
  NGC 4261 by Core Shift Measurements
  We report multifrequency phase-referenced observations of the nearby radio
galaxy NGC 4261, which has prominent two-sided jets, using the Very Long
Baseline Array at 1.4-43 GHz. We measured radio core positions showing
observing frequency dependences (known as "core shift") in both approaching
jets and counter jets. The limit of the core position as the frequency
approaches infinity, which suggests a jet base, is separated by 82$\pm$16
${\mu}$as upstream in projection, corresponding to (310$\pm$60)Rs (Rs:
Schwarzschild radius) as a deprojected distance, from the 43 GHz core in the
approaching jet. In addition, the innermost component at the counter jet side
appeared to approach the same position at infinity of the frequency, indicating
that cores on both sides are approaching the same position, suggesting a
spatial coincidence with the central engine. Applying a phase referencing
technique, we also obtained spectral index maps, which indicate that emission
from the counter jet is affected by free-free absorption (FFA). The result of
the core shift profile on the counter jet also requires FFA because the core
positions at 5-15GHz cannot be explained by a simple core shift model based on
synchrotron self-absorption (SSA). Our result is apparently consistent with the
SSA core shift with an additional disk-like absorber over the counterjet side.
Core shift and opacity profiles at the counter jet side suggest a two-component
accretion: a radiatively inefficient accretion flow at the inner region and a
truncated thin disk in the outer region. We proposed a possible solution about
density and temperature profiles in the outer disk on the basis of the radio
observation.
