The spectral state transition of Mkn 590, a potential link between AGNs and X-ray binaries?

TL;DR

This study documents a rare, real-time spectral state transition in the AGN Mkn 590, revealing parallels with X-ray binaries and supporting the idea of scale-invariant black hole accretion physics.

Contribution

It provides the first detailed, time-resolved observation of an AGN state transition analogous to X-ray binaries, linking accretion regimes across different black hole masses.

Findings

Mkn 590 exhibits a complete spectral state transition over ~10 years.

The X-ray loudness parameter shows a 'V'-shaped dependence on Eddington ratio.

The source broadly follows the fundamental plane of black hole activity with a flatter slope.

Abstract

The Seyfert galaxy Markarian 590 offers a rare glimpse into the dynamic life cycle of black hole accretion, captured across multiple wavelengths from the years 1975 to 2025. Using the decade-long multi-band observations from the Swift observatory, we capture a clear spectral state transition analogous to those seen in X-ray binaries but seldom observed in a single AGN. We track a complete AGN state transition in real time, as the source evolves from a faint, hard X-ray state to a bright, UV- and soft X-ray dominated phase. The X-ray loudness parameter $\alpha_{ox}$ follows a pronounced 'V'-shaped dependence on Eddington ratio $\lambda_{Edd}$, with a break at $\lambda_{Edd}$ = 0.021 +/- 0.008, coinciding with thresholds identified in population studies of changing-look quasars and X-ray binaries. Across this transition, Mkn 590 evolves through distinct accretion regimes in the Hardness Intensity Diagrams (HID): faint, flaring, transitional, and bright, on a timescale of $\sim$10 years, which is well below classical viscous timescales for a geometrically thin disk but in agreement with the propagation of thermal wavefronts in the inner disk. When placed on the Fundamental Plane of black hole activity, the source broadly follows the expected radio/X-ray mass scaling, though with a 70% flatter slope, pointing towards a persistent coronal-jet coupling even in radiatively efficient states. Together, our results establish Mkn 590 as a rare, time-resolved case of AGN state transitions and offer compelling evidence for scale-invariant accretion physics across the black hole mass spectrum.