Jet kinematics in the transversely stratified jet of 3C 84

Abstract

3C 84 (NGC 1275) is one of the brightest radio sources in the millimetre radio bands, which led to a plethora of very-long-baseline interferometry (VLBI) observations at numerous frequencies over the years. They reveal a two-sided jet structure, with an expanding but not well-collimated parsec-scale jet, pointing southward. High-resolution millimetre-VLBI observations allow the study and imaging of the jet base on a sub-parsec scale. This could facilitate the investigation of the nature of the jet origin, also in view of the previously detected two-railed jet structure and east-west oriented core region seen with RadioAstron at 22 GHz. We produced VLBI images of this core and inner jet region, observed over the past twenty years at 15, 43, and 86 GHz. We determined the kinematics of the inner jet and ejected features at 43 and 86 GHz and compared their ejection times with radio and γ-ray variability. For the moving jet features, we find an average velocity of β avg app = 0.055 − 0.22c (µ avg = 0.04 − 0.18 mas/yr). From the time-averaged VLBI images at the three frequencies, we measured the transverse jet width along the bulk flow. On the ≤ 1.5 parsec scale, we find a clear trend of the jet width being frequency dependent, with the jet being narrower at higher frequencies. This stratification is discussed in the context of a spine-sheath scenario, and we compare it to other possible interpretations. From quasi-simultaneous observations at 43 and 86 GHz, we obtain spectral index maps, revealing a time-variable orientation of the spectral index gradient due to structural variability of the inner jet.

A multi-band study and exploration of the radio wave–γ-ray connection in 3C 84

“…5. Features F3 and F4 (Paraschos et al 2022) might be connected to γ-ray flares; F5 does not seem to have a radio flux counterpart. 6.…”

“…The authors found that the major γ-ray flares seem to be associated with the merging and splitting of jet components (at 43 GHz) in the C3 region. Paraschos et al (2022) found three core-ejected features (cross-identified at 43 and 86 GHz), denoted as 'F3', 'F4', and 'F5', which appear associated with outbursts in the centimetre and millimetre flux. Specifically, F3, and F4 seem to correspond 2018) we also identified the flare called 'G1' 4 , with a point of ejection in 2013.9±0.1, which matches, within the error budget, the ejection time of a γ-ray flare associated with F3.…”

“…This data set (see Fig. 1) has already been published in Paraschos et al (2022) and we refer the interested reader to that work for further details. An overview of the light curve parameters is presented in Table 1.…”

“…Its core and milliarcsecond (mas) region have been imaged in great detail at many frequencies, both in total intensity (e.g. Giovannini et al 2018;Paraschos et al 2022) and polarisation (Kim et al 2019), revealing a limbbrightened structure (see for example Kramer & MacDonald 2021, for a discussion based on magneto-hydrodynamic simulations simulations (MHD)) and multiple features being ejected almost monthly (Paraschos et al 2022). Locating the exact position of the jet apex might offer an explanation for the complex structures observed in the core (e.g.…”

“…Locating the exact position of the jet apex might offer an explanation for the complex structures observed in the core (e.g. Giovannini et al 2018;Kim et al 2019;Oh et al 2022;Paraschos et al 2022). Estimates of the jet apex position in 3C 84 have been presented in the past; Giovannini et al (2018) suggested that the emission north of the 22 GHz core constitutes the sub-mas counterjet, and thus the jet apex must be within z core ∼ 30 µas of the 22 GHz VLBI core.…”