Clarify the role of fast magnetosonic propagation in disk fluctuations

Clarify the role of the fast magnetosonic speed in determining the inward and outward propagation of density fluctuations in radiation magnetohydrodynamic AGN disk simulations.

Background

The paper compares the measured propagation speeds of density fluctuations with both the local inflow speed and the fast magnetosonic speed. Although prior work associated fluctuation propagation with the fast magnetosonic speed, the simulated propagation speeds near the photosphere are substantially slower than the local fast magnetosonic speed and instead resemble the fast magnetosonic speed near the midplane.

This discrepancy leaves unresolved which physical speed governs the observed propagation and the corresponding long lags in AGN light curves. Resolving the issue would improve the interpretation of long-lag measurements as probes of disk structure and inflow physics.

References

Therefore, the role this speed plays in the propagation of fluctuations is less clear than the role of the inflow speed.

— Disk Structure May Determine AGN Variability and Explain the Accretion Disk Size Problem: No Broad Line Region Required  (2610.12462 - Secunda et al., 8 Oct 2026) in Appendix A, “Disk Structure”