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Constraints on the Circumnuclear Disk through Free-Free Absorption in the Nucleus of 3C 84 with KaVA and KVN at 43 and 86 GHz SCI
  • Kiyoaki Wajima; Motoki Kino; Nozomu Kawakatu;
  • 2020-05-21
  • ASTROPHYSICAL JOURNAL 895 1 : 35-1~35-8
The nearby bright radio galaxy 3C~84 at the center of the Perseus cluster is an ideal target to explore the jet in an active galactic nucleus and its parsec-scale environment. The recent research of Fujita \& Nagai revealed the existence of the northern counter-jet component (N1) located 2~mas north from the central core in very long baseline interferometer (VLBI) images at 15 and 43~GHz and they are explained by the free-free absorption (FFA) due to an ionized plasma foreground. Here we report a new quasi-simultaneous observation of 3C~84 with the Korean VLBI Network (KVN) at 86~GHz and the KVN and VLBI Exploration of Radio Astrometry Array (KaVA) at 43~GHz in 2016 February. We succeeded the first detection of N1 at 86~GHz and the data show that N1 still has an inverted spectrum between 43 and 86~GHz with its spectral index $\alpha$ ($S_{\nu} \propto \nu^{\alpha}$) of $1.19 \pm 0.43$, while the approaching lobe component has a steep spectrum with an index of $-0.54 \pm 0.30$. Based on the measured flux asymmetry between the counter and approaching lobes, we constrain the averaged number density of the FFA foreground $n_{\rm e}$ as $1.8 \times 10^{4}~{\rm cm^{-3}} \lesssim n_{\rm e} \lesssim 1.0 \times 10^{6}~{\rm cm^{-3}}$. Those results suggest that the observational properties of the FFA foreground can be explained by the dense ionized gas in the circumnuclear disk and/or assembly of clumpy clouds at the central $\sim 1$~pc region of 3C~84.