Monitoring of multi-frequency polarization of gamma-ray bright AGNs

We started two observing programs with the Korean VLBI Network (KVN) monitoring changes in the flux density and polarization of relativistic jets in gamma-ray bright AGNs simultaneously at 22, 43, 86, 129 GHz. One is a single-dish weekly-observing program in dual polarization with KVN 21-m diameter radio telescopes beginning in 2011 May. The other is a VLBI monthly-observing program with the three-element VLBI network at an angular resolution range of 1.0--9.2 mas beginning in 2012 December. The monitoring observations aim to study correlation of variability in gamma-ray with that in radio flux density and polarization of relativistic jets when they flare up. These observations enable us to study the origin of the gamma-ray flares of AGNs.Comment: 4 pages, 4 figures, Proceedings of the conference "The innermost regions of relativistic jets and their magnetic fields", Granada, Spai

PAGaN II: The Evolution of AGN Jets on Sub-Parsec Scales

We report first results from KVN and VERA Array (KaVA) VLBI observations obtained in the frame of our Plasma-physics of Active Galactic Nuclei (PAGaN) project. We observed eight selected AGN at 22 and 43 GHz in single polarization (LCP) between March 2014 and April 2015. Each source was observed for 6 to 8 hours per observing run to maximize the $uv$ coverage. We obtained a total of 15 deep high-resolution images permitting the identification of individual circular Gaussian jet components and three spectral index maps of BL Lac, 3C 111 and 3C 345 from simultaneous dual-frequency observations. The spectral index maps show trends in agreement with general expectations -- flat core and steep jets -- while the actual value of the spectral index for jets shows indications for a dependence on AGN type. We analyzed the kinematics of jet components of BL Lac and 3C 111, detecting superluminal proper motions with maximum apparent speeds of about $5c$. This constrains the lower limits of the intrinsic component velocities to $\sim0.98c$ and the upper limits of the angle between jet and line of sight to $\sim$20$\deg$. In agreement with global jet expansion, jet components show systematically larger diameters $d$ at larger core distances $r$, following the global relation $d\approx0.2r$, albeit within substantial scatter.Comment: 13 pages, 15 figures, 4 tables. To appear in JKAS (received 2015 August 31; accepted 2015 October 15). Note the PAGaN I companion paper by J.-Y. Kim et a

Long-term millimeter VLBI monitoring of M87 with KVN at milliarcsecond resolution: nuclear spectrum

We study the centimeter- to millimeter-wavelength synchrotron spectrum of the core of the radio galaxy M87 at $\lesssim0.8\,{\rm mas}~\sim110R_{s}$ spatial scales using four years of fully simultaneous, multi-frequency VLBI data obtained by the Korean VLBI Network (KVN). We find a core spectral index $\alpha$ of $\gtrsim-0.37$ ($S\propto \nu^{+\alpha}$) between 22GHz and 129GHz. By combining resolution-matched flux measurements from the Very Long Baseline Array (VLBA) at 15GHz and taking the Event Horizon Telescope (EHT) 230GHz core flux measurements in epochs 2009 and 2012 as lower limits, we find evidence of a nearly flat core spectrum across 15GHz and 129GHz, which could naturally connect the 230GHz VLBI core flux. The extremely flat spectrum is a strong indication that the jet base does not consist of a simple homogeneous plasma, but of inhomogeneous multi-energy components, with at least one component with the turn-over frequency $\gtrsim100$GHz. The spectral shape can be qualitatively explained if both the strongly (compact, optically thick at $>$100GHz) and the relatively weakly magnetized (more extended, optically thin at $<$100GHz) plasma components are colocated in the footprint of the relativistic jet.Comment: 5 pages, 5 figures, accepted for A&A letter