The millimeter-wave continuum spectrum of Centaurus A and its nucleus

TL;DR

This study measures the millimeter continuum spectrum of Centaurus A and its nucleus, revealing spectral steepening, core variability, and correlations with X-ray emissions, enhancing understanding of its radio emission mechanisms.

Contribution

It provides the first detailed millimeter spectrum of Centaurus A's nucleus and analyzes its variability and spectral properties over time.

Findings

Spectral index steepens above 5 GHz from -0.70 to -0.82.

Millimeter emission from the core is variable and correlates with high-energy X-ray variability.

Core spectral index varies from -0.3 in quiescence to steeper when brightened.

Abstract

In a study of the radio emission mechanism of the FR-I AGN NGC 5128 (Centaurus A)}, we have determined the centimeter and millimeter continuum spectrum of the whole Centaurus A radio source and measured the continuum emission from the active galxy nucleus at various times between 1989 and 2005 at frequencies between 86 GHz (3.5 mm) and 345 GHz (0.85 mm). The integral Cen A spectrum becomes steeper at frequencies above 5 GHz, where the spectral index changes from -0.70 to -0.82. Millimeter emission from the core of Centaurus A is variable, and correlates appreciably better with the 20-200 keV than the 2 - 10 keV X-ray variability. In its quiescent state, the core spectral index is -0.3, which steepens when the core brightens. The variability appears to be mostly associated with the inner nuclear jet components that have been detected in VLBI measurements. The densest nuclear components are optically thick below 45-80 GHz.