Pinpointing the jet apex of 3C 84

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 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.

Millimeter-VLBI observations of low-luminosity active galactic nuclei with source-frequency phase-referencing

“…Following the Equation (4) in Lobanov (1998), we could estimate the core-shift measure Ω r,22−44 = 0.13±0.06 [pc GHz] and Ω r,44−88 = 0.12 ± 0.10 [pc GHz], using the resultant power law index k r = 1.09. The magnetic field strength at the 88 GHz core is estimated to be 4.8 ± 2.4 G using the mean value of Ω r , by Equation (B.2) in Paraschos et al (2021) with a spectral index of −0.5, a Doppler factor of 2, a jet viewing angle of 18 • and an intrinsic jet opening angle of 63 • .6 at 88 GHz (Kim et al 2018). This is consistent with the estimated 1-30 G near event horizon by the Event Horizon Telescope at 230 GHz (Event Horizon Telescope Collaboration et al 2021).…”

A persistent double nuclear structure in 3C 84

“…From this we obtain a jet apex position of 54±9 𝜇as for N1, and 215±23 𝜇as for N2. From an independent core-shift analysis based on a single epoch image (May 2015), Paraschos et al (2021) recently reported a jet apex location 83±7 𝜇as north of the C1ab complex. This is close to the position, which we derive for N1.…”

“…caused by jet rotation). Since the measurements of Punsly et al (2021) are closer in time than our measurement, we combine their speed measurement with the results from Paraschos et al (2021) to derive a jet viewing angle of 17 • . In all cases, the viewing angle in the C1 region is less than about 35 • and could be yet smaller than this limit.…”

“…The differences are surprising, given that the space-VLBI observations are of higher resolution than ours. The recent work by Paraschos et al (2021) places the 3 mm VLBI core 0.35 mas upstream of the 15 GHz core. So we consider it unlikely that the C1 region is the true jet base (see Section 4.3).…”

“…Green crosses are CLEAN components counted on the counter-jet side, blue crosses are CLEAN components counted on the jet side and grey crosses are CLEAN components that were not included in the analysis.The dashed black boxes have a size of 0.3 mas × 0.3 mas and the CLEAN components within them are summed in order to perform the analysis. The dividing lines are, from the left, N1, N2 (as described in Section 7 and Figure7) and P2021, which is the approximate location of the jet apex as measured byParaschos et al (2021). The flux density of summed CLEAN components in each box is indicated next to the boxes.…”