Multi-waveband polarimetric observations of NRAO 530 on parsec-scale
  We report on multi-bands VLBA polarimetric observations of NRAO 530 in
February 1997. Total intensity, EVPA distributions at all these frequencies are
presented. Model fitting has been performed, from which the fitted southmost
component A is confirmed as the core of the radio structure with relatively
high brightness temperature and hard spectrum between 15 and 43 GHz in
comparison with the central component B of dominant flux. The relatively high
degree of polarization for the component A may arise from its complex radio
structure, which is resolvable at 86 GHz. As a contrast, the component B shows
a well fitted power-law spectrum with the spectral index of about -0.5, and a
linear correlation between EVPAs and wavelength square with the observed RM of
about -1062 rad m^{-2}, indicating its structural singleness. Assuming that the
component B has a comparable degree of polarization without depolarization at
these frequencies, the decrease in fractional polarization with wavelength
mainly results from opacity and Faraday rotation, in which the opacity plays
quite a large part of role. A spine-sheath like structure in fractional
polarization is detected covering almost the whole emission region at 5 and 8
GHz. The linear polarization at 5 GHz shows 3 separate polarized emission
regions with alternately aligned and orthogonal polarization vectors down the
jet. The polarization goes to zero between the top two regions, with the
highest polarization level occurring at the top and bottom. The 5 and 8 GHz
images show EVPA changes across the width of the jet as well as along the jet.
These complex polarimetric properties can be explained in terms of either the
presence of a large helical magnetic field or tangled magnetic fields
compressed and sheared down the jet.
