Letter to the Editor

Universality of coronal properties in accreting black holes across mass and accretion rate

¹ Université Paris Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
e-mail: sudip.chakraborty@cea.fr
² INAF-IAPS, Via del Fosso del Cavaliere 100, 00133 Rome, Italy
³ Department of Physics, University of Rome ‘Tor Vergata’, Via della Ricerca Scientifica 1, 00133 Rome, Italy
⁴ INAF – Astronomical Observatory of Rome, Via Frascati 33, 00078 Monte Porzio Catone, Rome, Italy
⁵ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁶ NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁷ INFN – Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
⁸ Universidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Departamento de Astronomia, São Paulo, SP 05508-090, Brazil
⁹ Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), Stanford University, Stanford, CA 94305, USA
¹⁰ Inter University Centre for Astronomy and Astrophysics, Post bag 4, Ganeshkhind, Pune, India

Received: 14 June 2023
Accepted: 2 August 2023

Abstract

Aims. Through their radio loudness, lack of thermal UV emission from the accretion disk, and power-law dominated spectra, low-luminosity active galactic nuclei (LLAGNs) display similarity with the hard state of stellar-mass black hole X-ray binaries (BHBs). In this work we perform a systematic hard X-ray spectral study of a carefully selected sample of unobscured LLAGNs using archival NuSTAR data to understand the central engine properties in the lower accretion regime.

Methods. We analyzed the NuSTAR spectra of a sample of 16 LLAGNs. We modeled the continuum emission with detailed Comptonization models.

Results. We find a strong anti-correlation between the optical depth and the electron temperature of the corona, previously also observed in the brighter Seyferts. This anti-correlation is present irrespective of the shape of the corona, and the slope of this anti-correlation in the log space for LLAGNs (0.68–1.06) closely matches that of the higher accretion rate Seyferts (0.55–1.11) and hard state of BHBs (∼0.87). This anti-correlation may indicate a departure from a fixed disk-corona configuration in radiative balance.

Conclusions. Our result, therefore, demonstrates a possible universality in Comptonization processes of black hole X-ray sources across multiple orders of magnitude in mass and accretion rate.