The 1.4 mm core of Centaurus A: First VLBI results with the South Pole Telescope

TL;DR

This paper reports the first very-long-baseline interferometry (VLBI) observations of Centaurus A at 1.4 mm wavelength using the South Pole Telescope, revealing detailed core properties and size constraints near the black hole.

Contribution

It presents the first VLBI measurement of Cen A at 1.4 mm, providing high-resolution core size and brightness temperature estimates close to the theoretical equipartition limit.

Findings

Measured correlated flux density at 1.4 mm on a 7000 km baseline.

Inferred core brightness temperature of 1.4 x 10^11 K.

Derived an upper limit to the core size of 34 microarcseconds.

Abstract

Centaurus A (Cen A) is a bright radio source associated with the nearby galaxy NGC 5128 where high-resolution radio observations can probe the jet at scales of less than a light-day. The South Pole Telescope (SPT) and the Atacama Pathfinder Experiment (APEX) performed a single-baseline very-long-baseline interferometry (VLBI) observation of Cen A in January 2015 as part of VLBI receiver deployment for the SPT. We measure the correlated flux density of Cen A at a wavelength of 1.4 mm on a $\sim$7000 km (5 G$\lambda$) baseline. Ascribing this correlated flux density to the core, and with the use of a contemporaneous short-baseline flux density from a Submillimeter Array observation, we infer a core brightness temperature of $1.4 \times 10^{11}$ K. This is close to the equipartition brightness temperature, where the magnetic and relativistic particle energy densities are equal. Under the assumption of a circular Gaussian core component, we derive an upper limit to the core size $\phi = 34.0 \pm 1.8~\mu\textrm{as}$, corresponding to 120 Schwarzschild radii for a black hole mass of $5.5 \times 10^7 M_{\odot}$.