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

TL;DR

This study reveals a universal anti-correlation between optical depth and electron temperature in the coronae of accreting black holes, spanning from stellar-mass binaries to supermassive black holes, indicating similar underlying physics across different scales.

Contribution

It demonstrates a universal anti-correlation in coronal properties of black holes across different masses and accretion rates, based on systematic NuSTAR spectral analysis of LLAGN.

Findings

Anti-correlation between optical depth and electron temperature observed.

The anti-correlation slope in LLAGN matches that in Seyferts and BHBs.

Results suggest a universal Comptonization process in black hole accretion.

Abstract

Through their radio loudness, lack of thermal UV emission from the accretion disk and power-law dominated spectra, Low Luminosity AGN (LLAGN) 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 LLAGN using archival $NuSTAR$ data, to understand the central engine properties in the lower accretion regime. We analyze the $NuSTAR$ spectra of a sample of 16 LLAGN. We model the continuum emission with detailed Comptonization models. 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 LLAGN (0.68-1.06) closely matches that of the higher accretion rate Seyferts (0.55-1.11) and hard state of BHBs ($\sim$0.87). This anti-correlation may indicate a departure from a fixed disk-corona configuration in radiative balance. 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.