ALMA observation of an evolving magnetized corona in the radio-quiet changing-state active galactic nucleus NGC 1566

¹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
² Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People's Republic of China
³ Joint ALMA Observatory, Avenida Alonso de Cordova 3107, Vitacura 7630355, Santiago, Chile
⁴ Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
⁵ Interdisciplinary Theoretical & Mathematical Science Program (iTHEMS), RIKEN, 2-1 Hirosawa, 351-0198, Japan
⁶ Kavli Institute for the Physics and Mathematics of the Universe (WPI), UTIAS, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan
⁷ RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama, Saitama 351-0198, Japan
⁸ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
⁹ School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
¹⁰ Eureka Scientific, 2452 Delmer Street Suite 100, Oakland, CA 94602-3017, USA
¹¹ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA

^⋆ Corresponding author: arghajit.jana@mail.udp.cl

Received: 12 March 2025
Accepted: 19 May 2025

Abstract

The origin of compact millimeter (mm) continuum emission from radio-quiet active galactic nuclei (RQAGNs) remains not fully understood. Changing-state AGNs (CSAGNs) exhibit rapid and strong variability, which allows us to investigate the origin of the mm emission. We present here the results of a first study of the mm continuum variability of a CSAGN using archival ALMA band 6 (∼230 GHz) observations of NGC 1566 obtained from 2014 to 2023. We find a positive correlation between the mm and X-ray flux with an intrinsic scatter of 0.05 dex (1σ), suggesting a common origin. The mm spectral index (α_mm) is found to be in the range of 0.13±0.38 to −0.26±0.53, consistent with a compact optically thick synchrotron source. No significant correlation was found between the α_mm and the mm flux. The mm/X-ray ratio also shows no clear link to the Eddington ratio but is higher in the low-accretion state. We discuss several scenarios about the origin of the mm emission in NGC 1566. We find that synchrotron emission in the magnetized X-ray corona appears to be the most probable origin of the mm emission, confirming that mm emission can be used as a tracer of AGN activity in RQAGNs.