Volume 650, June 2021
|Number of page(s)||8|
|Section||Letters to the Editor|
|Published online||24 June 2021|
Letter to the Editor
Pinpointing the jet apex of 3C 84
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, Bonn, Germany
2 Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 30455, Korea
Accepted: 8 June 2021
Nearby radio galaxies that contain jets are extensively studied with very long baseline interferometry (VLBI), addressing jet launching and the physical mechanisms at play around massive black holes. 3C 84 is unique in this regard because the combination of its proximity and large super massive black hole mass provides a high spatial resolution to resolve the complex structure at the jet base. For 3C 84, an angular scale of 50 μas corresponds to 200−250 Schwarzschild radii (Rs). Recent RadioAstron VLBI imaging at 22 GHz has revealed an east-west elongated feature at the northern end of the VLBI jet, which challenges past interpretations. Here we propose instead that the jet apex is not located within the 22 GHz VLBI core region but more upstream in the jet. We base our arguments on a 2D cross-correlation analysis of quasi-simultaneously obtained VLBI images at 15, 43, and 86 GHz, which measures the opacity shift of the VLBI core in 3C 84. With the assumption of the power-law index (kr) of the core shift being set to 1, we find the jet apex to be located 83 ± 7 μas north (upstream) of the 86 GHz VLBI core. Depending on the assumptions for kr and the particle number density power-law index, n, we find a mixed toroidal-poloidal magnetic field configuration, consistent with a region that is offset from the central engine by about 400–1500 Rs. The measured core shift is then used to estimate the magnetic field strength, which amounts to B = 1.80−4.0 G near the 86 GHz VLBI core. We discuss some physical implications of these findings.
Key words: galaxies: individual: 3C 84 (NGC 1275) / galaxies: jets / galaxies: active / techniques: interferometric / techniques: high angular resolution
© G. F. Paraschos et al. 2021
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Open Access funding provided by Max Planck Society.
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