A magnetically-supported disk-corona model for Changing-Look AGN transitions

Abstract

Changing-Look Active Galactic Nuclei (CLAGN) undergo dramatic spectral and luminosity transitions on timescales of months to a few years -- orders of magnitude shorter than the viscous timescale of a standard $\alpha$-disk at the radii where the optical/UV continuum is generated, for typical supermassive black hole masses. We show that a magnetically supported disk-corona model reproduces \emph{both} the observed Eddington ratio at which changing event occurs and the observed transition duration. Using the \texttt{diskvert} code, which solves the steady vertical structure under simultaneous gas, radiation and magnetic pressure support with a self-consistent warm corona, we (i) construct thermal-viscous S-curves, and (ii) calculate the integrated thermal timescale together with the front propagation timescale. We compute a large grid of models of different black hole masses, Eddington ratios, magnetic viscosities, and disk radii, showing that magnetized disks push the S-curve knee down to an Eddington ratio of $ \approx 0.01-0.03$, and introduce a new stable branch of high luminosity solutions, while the limit-cycle timescale enters the months-to-years range for $M_\mathrm{BH} = 10^{7}-10^{9}\,\mathrm{M_\odot}$. Confronted with a sample of five CLAGN (Mkn 590, NGC 1566, IRAS 23226$-$3843, Mkn 1018, NGC 2617), the model jointly reproduces the empirical Eddington rates and the observed event durations only when the inner disk is strongly magnetized. The case of Mkn 590 is especially constraining: the recent tightly-determined transition Eddington ratio is matched by a highly magnetized disk-corona flow at small radii.