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A long-standing question in active galactic nucleus (AGN) research is how thecorona is heated up to produce X-ray radiation much stronger than that arisingfrom the viscous heating within the corona. In this paper, we carry outdetailed investigations of magnetic-reconnection heating to the corona,specifically, studying how the disc and corona are self-consistently coupledwith the magnetic field, and how the emergent spectra depend on the fundamentalparameters of AGN. It is shown that diverse spectral shapes and luminositiesover a broad bandpass from optical to X-ray can be produced from the coupleddisc and corona within a limited range of the black hole mass, accretion rateand magnetic field strength. The relative strength of X-ray emission withrespect to optical/ultraviolet (UV) depends on the strength of the magneticfield in the disc, which, together with accretion rate, determines the fractionof accretion energy transported and released in the corona. This refineddisc-corona model is then applied to reproduce the broad-band spectral energydistributions (SEDs) of a sample of 20 bright local AGNs observedsimultaneously in X-ray and optical/UV. We find that, in general, the overallobserved broad-band SEDs can be reasonably reproduced, except for rather hardX-ray spectral shapes in some objects. The radiation pressure-dominant region,as previously predicted for the standard accretion disc in AGN, disappears forstrong X-ray sources, revealing that AGN accretion discs are indeed commonlystable as observed. Our study suggests the disc-corona coupling model involvingmagnetic fields to be a promising approach for understanding the broad-bandspectra of bright AGNs.

Magnetic-reconnection-heated corona in active galactic nuclei: refined disc-corona model and application to broad-band radiation