The Dense Plasma Torus around the Nucleus of an Active Galaxy NGC1052

Abstract

A subparsec-scale dense plasma torus around an active galactic nucleus is unveiled. We report on very-long-baseline interferometry (VLBI) observations at 2.3, 8.4, and 15.4 GHz towards the active galaxy NGC1052. The convex spectra of the double-sided jets and the nucleus imply that synchrotron emission is obscured through free–free absorption (FFA) by the foreground cold dense plasma. A trichromatic image was produced to illustrate the distribution of the FFA opacity. We found a central condensation of the plasma which covers about 0.1 pc and 0.7 pc of the approaching and receding jets, respectively. A simple explanation for the asymmetric distribution is the existence of a thick plasma torus perpendicular to the jets. We also found an ambient FFA absorber, whose density profile can be ascribed to a spherical distribution of the isothermal King model. The coexistence of torus-like and spherical distributions of the plasma suggests a transition from radial accretion to rotational accretion around the nucleus.

Positional coincidence between an H2O maser and a plasma torus in NGC 1052

“…The spectrum index exceeds the theoretical limit for synchrotron self-absorption (α = 2.5). The highly inverted spectrum of the inner edge of the jets implies that synchrotron emission is obscured through free-free absorption (FFA) by the foreground dense plasma, and it is consistent with past multi-frequency observations (Kameno et al 2001, Vermeulen et al 2003and Kadler et al 2004.…”

“…The spectrum index exceeds the theoretical limit for synchrotron self-absorption (α = 2.5). The highly inverted spectrum of the inner edge of the jets implies that synchrotron emission is obscured through free-free absorption (FFA) by the foreground dense plasma, and it is consistent with past multi-frequency observations (Kameno et al 2001, Vermeulen et al 2003and Kadler et al 2004.Fitting the continuum spectrum at 15-43 GHz to FFA model (S ν ∝ ν −α exp (−τ ν −2.1 )), we obtained FFA opacity (τ ) distributions along the jet axis (figure 1), which reveals that a high opacity (τ > 1000) is found in the inner edge. It implies that the dense cold plasma covers ∼2 mas in the inner edge of the jets, where the central engine is supposed to exist.…”

Mm-wave HCO+, HCN and CO absorption toward NGC 1052

“…Kameno et al (2001) have modelled an asymmetric electron distribution having a peak free-free opacity of 300 toward the core at 15.4 GHz. Scaling as ν −2.1 , the peak opacity at 89-115 GHz would be only 4-7.…”

The twin-jet system in NGC 1052: VLBI-scrutiny of the obscuring torus

“…Radio observations of the low-luminosity active galactic nucleus (AGN) NGC 1052 with Very Long Baseline Interferometry (VLBI) at multiple frequencies have revealed the presence of a dense circumnuclear absorber, which obscures the very center of this elliptical galaxy (Kellermann et al 1999;Kameno et al 2001;Vermeulen et al 2003). Indeed, the standard model of AGNs predicts the existence of an obscuring torus, whose inner surface is expected to be photo-ionized by illumination from the accretion disk.…”