Pinpointing the jet apex of 3C 84

G F Paraschos,T P Krichbaum,J.-Y Kim,J A Zensus

doi:10.1051/0004-6361/202140776

G F Paraschos, T P Krichbaum + Show 2 more

Open Access

https://doi.org/10.1051/0004-6361/202140776

Copy DOI

Abstract

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 forkrand 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 400–1500Rs. The measured core shift is then used to estimate the magnetic field strength, which amounts toB= 1.80−4.0 G near the 86 GHz VLBI core. We discuss some physical implications of these findings.

Highlights

The jet in 3C 84 is launched via magneto-centrifugal acceleration from the accretion disk – the Blandford & Payne (1982) (BP) model – or via energy extraction directly from the ergosphere of the spinning central black hole – the Blandford & Znajek (1977) (BZ) model
In this Letter we have studied the spectral index and core shift of 3C 84
The analysis of the core shift reveals that the jet apex is located north-west of the very long baseline interferometry (VLBI) core at 86 GHz, displaced by 76−90 μas, with the distance from the core as a function of the frequency, following Eq (2)

Summary

Introduction

The jet in 3C 84 is launched via magneto-centrifugal acceleration from the accretion disk – the Blandford & Payne (1982) (BP) model – or via energy extraction directly from the ergosphere of the spinning central black hole – the Blandford & Znajek (1977) (BZ) model. Total intensity and polarisation imaging results have already been published; for the details we refer to Kim et al (2019) In this follow-up paper we used the same data but focus on the spectral properties and opacity shift in the VLBI core region. At 43 GHz and 86 GHz, the intensity peak is at the northernmost region of the jet and no additional shift is necessary In this procedure and at all frequencies, we fitted a circular Gaussian to the VLBI core to identify their position shifts from the phase centres, finding

Implications of the jet apex location

Magnetic field strength and topology

Conclusions