Disk corona interaction: mechanism for the disk truncation and spectrum change in low luminosity AGN

TL;DR

This paper models the interaction between the disk and corona in low luminosity AGN, showing how magnetic fields influence disk truncation and spectral features, aligning with observed infrared emissions.

Contribution

It extends the disk evaporation model by incorporating magnetic effects, revealing their impact on disk truncation radius and accretion rates in LLAGN.

Findings

Truncation radius is smaller with magnetic effects included.

Critical accretion rate for disk truncation is insensitive to magnetic fields.

Spectral fits of LLAGN are consistent with the model's predictions.

Abstract

The truncation of an optically thick, geometrically thin accretion disk is investigated in the context of low luminosity AGN (LLAGN). We generalize the disk evaporation model used in the interpretative framework of black hole X-ray binaries by including the effect of a magnetic field in accretion disks surrounding supermassive black holes. The critical transition mass accretion rate for which the disk is truncated is found to be insensitive to magnetic effects, but its inclusion leads to a smaller truncation radius in comparison to a model without its consideration. That is, a thin viscous disk is truncated for LLAGN at an Eddington ratio less than 0.03 for a standard viscosity parameter ($\alpha = 0.3$). An increase of the viscosity parameter results in a higher critical transition mass accretion rate and a correspondingly smaller truncation distance, the latter accentuated by greater magnetic energy densities in the disk. Based on these results, the truncation radii inferred from spectral fits of LLAGN published in the literature are consistent with the disk evaporation model. The infrared emission arising from the truncated geometrically thin accretion disks may be responsible for the red bump seen in such LLAGN.