The Core-like Nature of HST-1 in the M87 Jet
  We investigate the total flux density, spectral, polarization, and Faraday
rotation variability of HST-1 in the M87 jet during the outburst from 2003 to
2007 through multi-epoch VLA observations at 8, 15, and 22 GHz. Contrary to the
general case for blazars, the flux densities of HST-1 rise earlier at lower
frequencies from radio to X-ray, and the spectra are softening with the growth
of outburst, indicating that the newly emerging subcomponents within HST-1 have
relatively steep spectra. In particular, the intrinsic EVPA varies
monotonically by $\sim90^\circ$ at the 3 wavebands during the period, and all
but the stationary subcomponent in the eastern end of HST-1 move downstream
superluminally deviating divergently from the overall jet direction, with the
motion of the outmost subcomponent bending from one side of the jet axis to
another. These strongly argue for the presence of helical magnetic fields
around HST-1, which is also supported by the fact that the subcomponents might
be accelerated in this region. The fractional polarization is relatively low in
the rising stage, and in the decaying stage the polarization levels are almost
comparable at the 3 wavebands. In view of the quite large RM values, Faraday
rotation is expected to occur dominantly external to HST-1 in the decaying
stage, which is well supported by the presence of diffuse emission around
HST-1, and consistent with the scenario that RM decrease gets slower with time.
