A Study on Plasma-Physical Properties of Radio-Bright Nearby AGN with KVN and KaVA

Abstract

Nearby bright Active Galactic Nuclei (AGN) are important sources to study relativistic jets of complex structure in detail. Improving our physical understanding about launching, acceleration, and propagation of the structured outflow requires information on (i) internal conditions of the plasma such as particle densities and degree of turbulence and (ii) the strength and geometry of magnetic fields pervading the jets. Multi-frequency VLBI observations – especially with polarimetry – have been crucial to spatially resolve the jet structures on different scales and to extract detailed physical information from emission features. In this work, we show results and analysis of the first-epoch data obtained from the Plasma physics of Active Galactic Nuclei project, which is an ongoing program aimed at studying spectral and polarimetric properties of parsec-scale radio jets in AGN. Seven radio-bright nearby AGN were observed at frequencies of 22, 43, 86, and 129 GHz in dual polarization with Korean VLBI Network (KVN). Also, for higher angular resolution, the targets are observed with a joint VLBI array made by KVN and VERA (KaVA) at 43 GHz. Total intensity source maps are obtained successfully for most of the observations. At 22 and 43 GHz KVN detected polarizations from 3C 111, 3C 120, BL Lac, and DA 55. The observations have three main results. Firstly, apparent brightness temperatures of the jet components and radio cores measured by KaVA are found to be comparable to or lower than the plasma equipartition temperature (∼ 10^11K). This indicates that the observed unpolarized synchrotron radiation can be explained without exotic emission mechanisms such as inverse Compton radiation. Secondly, as for polarization properties, observed degree of polarization mpol for each source was relatively low, i.e. less than 10%. By simple calculation, it is shown that strong turbulences in the jet is able to suppress the intrinsic mpol. In the light of this model, we analyzed an exceptionally high degree of polarization detected from an extended blob of BL Lac at 43 GHz (mpol ∼ 40%). In order to observe the high level of polarization the magnetic field configuration in the jet component should be highly ordered by additional physical processes. Assuming that there is a transverse shock front traveling to the downstream of a jet, we found that combination of properties of the shock front such as its observed orientation and the shock strength nshocked/nunshocked can explain the high degree of polarization. Finally, by using the fluxes of the cores and jets obtained over a wide range of frequencies by KVN, spectral indices a of the components are measured. Interestingly, spectral index of the core at high frequencies (i.e. a86,129GHz) became quite steep. If other side effects such as (a) coherence loss of the radio emissions at high frequencies or (b) different UV coverages of KVN at the high frequencies do not affect the data significantly, the observed steepening pattern implies that the AGN radio cores at mm-wavelengths could have different characteristics compared to the cmwavelength cores. Clearly, the multi-frequency polarimetric data obtained by KVN and KaVA revealed complicated spatial and spectral evolution pattern of the jets. Second- and third-epoch data from the Plasma physics of Active Galactic Nuclei project with the VLBI arrays in East Asia will probe spectral and polarimetric properties in more detail.