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

Abstract

Using the decade-long multi-band \textit{Swift} observations of the changing look AGN Markarian~590, we capture a clear state transition associated with its recent changing look event. Using the X-ray loudness parameter $\aox$, we track this evolution in real time as a probe of the disk-corona geometry. The $\aox$ follows a pronounced \lq V\rq-shaped dependence on Eddington ratio $\eddfrac$, with a statistically significant break at $\eddfrac =0.021\pm0.008$, consistent with thresholds identified in population studies of changing look quasars. This behavior is indicative of a change in the inner accretion flow, from a truncated disk with a dominant hot corona at low accretion rates to an inward-extending disk with enhanced UV emission and a prominent warm Comptonizing layer at higher rates. Independent UV and X-ray Eddington ratio tracers also show consistent breaks at $\eddfrac\sim0.004$. Mkn~590 evolves through distinct phenomenological accretion phases: from a faint, hard X-ray dominated state, through a flaring phase, to a bright, UV/soft X-ray dominated phase and exhibiting variability on month-, year-, and decade-long timescales. This overall evolution is shorter than classical viscous timescales but broadly consistent with propagating thermal fronts in the accretion disk. We also found a declining radio-to-X-ray luminosity ratio with increasing $\eddfrac$, indicating a relative suppression of radio emission as the disk becomes more dominant with respect to the X-ray corona. Together, these results establish Mkn~590 as a rare, time-resolved case of an AGN undergoing a state transition and suggest that its accretion physics is broadly analogous to that observed in XRBs.