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Low-luminosity active galactic nuclei (LLAGN) share characteristics with the hard state of stellar-mass black hole X-ray binaries (BHBs), particularly in their radio loudness, lack of thermal UV emission from an accretion disk, and spectra dominated by power-law emission. In this chapter, we present a detailed hard X-ray spectral analysis of a carefully selected sample of unobscured LLAGN using archival NuSTAR data, aiming to investigate the properties of their central engines in the low-accretion regime. By applying physically motivated Comptonization models to the continuum emission, we discover a pronounced anti-correlation between the optical depth and electron temperature in the corona. This trend, similar to what has been observed in more luminous Seyferts, appears to hold across various coronal geometries. The slope of the anti-correlation in logarithmic space for LLAGN (ranging from 0.68 to 1.06) is remarkably consistent with that seen in Seyferts (0.55–1.11) and the hard state of BHBs (around 0.87). This behavior likely stems from the Compton thermostat mechanism and also suggests deviations from a static disk-corona radiative configuration. Our results imply a potential universality in the Comptonization process across black hole systems spanning a wide range of masses and accretion rates. We also explore the implications of the Compton thermostat theory to account for this anti-correlation in LLAGN and discuss open questions that remain.

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Universality of Coronal Properties in Accreting Black Holes

  • 会议论文

  • 发表日期：

    2025年

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  • Sudip Chakraborty,
  • Ajay Ratheesh,
  • Francesco Tombesi,
  • Rodrigo Nemmen,
  • Srimanta Banerjee

摘要

Low-luminosity active galactic nuclei (LLAGN) share characteristics with the hard state of stellar-mass black hole X-ray binaries (BHBs), particularly in their radio loudness, lack of thermal UV emission from an accretion disk, and spectra dominated by power-law emission. In this chapter, we present a detailed hard X-ray spectral analysis of a carefully selected sample of unobscured LLAGN using archival NuSTAR data, aiming to investigate the properties of their central engines in the low-accretion regime. By applying physically motivated Comptonization models to the continuum emission, we discover a pronounced anti-correlation between the optical depth and electron temperature in the corona. This trend, similar to what has been observed in more luminous Seyferts, appears to hold across various coronal geometries. The slope of the anti-correlation in logarithmic space for LLAGN (ranging from 0.68 to 1.06) is remarkably consistent with that seen in Seyferts (0.55–1.11) and the hard state of BHBs (around 0.87). This behavior likely stems from the Compton thermostat mechanism and also suggests deviations from a static disk-corona radiative configuration. Our results imply a potential universality in the Comptonization process across black hole systems spanning a wide range of masses and accretion rates. We also explore the implications of the Compton thermostat theory to account for this anti-correlation in LLAGN and discuss open questions that remain.