TANAMI: Tracking active galactic nuclei with austral milliarcsecond interferometry

III. First-epoch S band images^★

¹ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: pbenke@mpifr-bonn.mpg.de
² Julius Maximilians University Würzburg, Faculty of Physics and Astronomy, Institute for Theoretical Physics and Astrophysics, Chair of Astronomy, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
³ NASA HQ, 300 E St SW, Washington, DC 20546-0002, USA
⁴ CSIRO Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
⁵ CSIRO Space and Astronomy, Canberra Deep Space Communications Complex, PO Box 1035, Tuggeranong, ACT 2901, Australia
⁶ School of Natural Sciences, University of Tasmania, Private Bag 37, Hobart, Tasmania 7001, Australia
⁷ Hartebeesthoek Radio Astronomy Observatory, PO Box 443, 1740 Krugersdorp, South Africa
⁸ Space Operations New Zealand Ltd, Hargest House, PO Box 1306, Invercargill 9840, New Zealand

Received: 29 August 2023
Accepted: 14 October 2023

Abstract

Context. With the emergence of very high energy astronomy (VHE; E > 100 GeV), new open questions were presented to astronomers studying the multi-wavelength emission from blazars. Answers to these open questions, such as the Doppler crisis, and finding the location of the high-energy activity have eluded us thus far. Recently, quasi-simultaneous multi-wavelength monitoring programs have shown considerable success in investigating blazar activity.

Aims. Such quasi-simultaneous observations across the electromagnetic spectrum became possible thanks to the launch of the Fermi Gamma-ray Space Telescope in 2008. In addition, with very long baseline interferometry (VLBI) observations, we can resolve the central parsec region of active galactic nuclei (AGN) and compare morphological changes to γ-ray activity in order to study high-energy-emitting blazars. To achieve our goals, we need sensitive, long-term VLBI monitoring of a complete sample of VHE-detected AGN.

Methods. We performed VLBI observations of TeV-detected AGN and high-likelihood neutrino associations as of December of 2021 with the Long Baseline Array (LBA) and other southern-hemisphere radio telescopes at 2.3 GHz.

Results. In this paper, we present first light TANAMI S-band images, focusing on the TeV-detected subsample of the full TANAMI sample. In addition to these VHE-detected sources, we show images of two flux density calibrators and two additional sources included in the observations. We study the redshift, 0.1–100 GeV photon flux, and S-band core brightness temperature distributions of the TeV-detected objects, and find that flat-spectrum radio quasars and low-synchrotron-peaked sources on average show higher brightness temperatures than high-synchrotron-peaked BL Lacs. Sources with bright GeV γ-ray emission also show higher brightness temperature values than γ-low sources.

Conclusions. Long-term monitoring programs are crucial for studying the multiwavelength properties of AGN. With the successful detection of even the faintest sources, with flux densities below 50 mJy, future work will entail kinematic analysis and spectral studies both at 2.3 and 8.4 GHz to investigate the connection between the radio and γ-ray activity of these objects.