Abstract
Context. With the increase in the sensitivity and resolution of radio interferometry within recent years, the study of the collimation and acceleration region of extragalactic jets in active galactic nuclei (AGN) has come into focus. Whereas a large fraction of AGN jets show a change from parabolic to conical collimation profile around the Bondi radius, there is a small number of sources that display a deviation from this standard picture, including the radio galaxy NGC 1052. Aims. We study the jet width profile, which provides valuable information about the interplay between the central engine and accretion disk system and the formation, acceleration, and collimation of the jets. Methods. We observed the double-sided, low-radio-power active galaxy NGC 1052 at six frequencies with the VLBA in 2017 and at 22 GHz with RadioAstron in 2016. These data are combined with archival 15, 22, and 43 GHz multi-epoch VLBA observations. From ridge-line fitting we obtained width measurements along the jet and counter-jet which were fitted with single and broken power laws. Results. We find a clear break point in the jet collimation profile at ∼104 RS (Schwarzschild radii). Downstream of the break, the collimation is conical with a power-law index of 1.0 − 1.2 (cylindrical 0; parabolic 0.5; conical 1) for both jets. On the other hand, the upstream power-law index of 0.36 for the approaching jet is neither cylindrical nor parabolic, and the value of 0.16 for the receding jet suggests this latter is close-to cylindrical. For both jets we find a large opening angle of ∼30° at a distance of ∼103 RS and well-collimated structures with an opening angle of < 10° downstream of the break. Conclusions. There are significant differences between the upstream collimation profiles of the approaching (eastern) and receding (western) jets. Absorption or scattering in the surrounding torus as well as an accretion wind may mimic a cylindrical profile. We need to increase the observing frequencies, which do not suffer from absorption, in order to find the true jet collimation profile upstream of 104 RS.
Highlights
The most prominent features of radio-loud active galactic nuclei (AGN; see e.g. Zensus 1997) are their relativistic jets (Blandford & Königl 1979)
On November 5, 2016, NGC 1052 was observed with the 10m Space Radio Telescope (SRT) Spekt-R of the RadioAstron space-Very Long Baseline Interferometry (VLBI) mission (Kardashev et al 2012) and a large ground array consisting of antennas from the Very-Long-Baseline Array (VLBA), Very Large Array (VLA), European VLBI Network (EVN), and the LongBaseline Array (LBA) at 22 GHz
Deviations of the apparent opening angle and width with frequency upstream of the break point can be explained by differential expansion, which is required by magnetic acceleration (Vlahakis & Königl 2004; Komissarov et al 2009; Komissarov 2012; Anglés-Castillo et al 2021)
Summary
The most prominent features of radio-loud active galactic nuclei (AGN; see e.g. Zensus 1997) are their relativistic jets (Blandford & Königl 1979). The high resolution provided by Very Long Baseline Interferometry (VLBI) at millimetre (mm) wavelengths (see Boccardi et al 2017), in this case by the Global mmVLBI array (GMVA), reveals very small transversal jet widths (e.g. M 87, Kim et al 2018) and wide jet bases (e.g. Cyg A, 3C 84 Giovannini et al 2018; Boccardi et al 2016a) This suggests that jet launching through the ergosphere of the black hole and by the accretion disk play an important role in jet formation (Blandford & Payne 1982; Blandford & Znajek 1977). On the other hand, based on a stacked 15 GHz Very-Long-Baseline Array (VLBA) image, Kovalev et al (2020) found a power-law index closer to a parabolic collimation profile for the upstream jet of k = 0.391 ± 0.048.
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