A break in the X-ray loudness of Markarian 590: evidence for an AGN spectral state transition?

Abstract

Using decade-long multi-band {\it Swift} observations of the changing-look AGN Markarian 590, we identify a clear break in the dependence of the X-ray loudness parameter $\alpha_{\rm ox}$ on the source accretion rate. This break could be potential evidence for an accretion state transition, analogous to that observed for X-ray binaries. The $\alpha_{\rm ox}$ follows a pronounced 'V'-shaped dependence on Eddington ratio $\lambda_{\rm Edd}$, with a statistically significant break at $\lambda_{\rm Edd} =0.021\pm0.008$, consistent with the Eddington-ratio threshold associated with changing-look events in 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. The UV and X-ray Eddington ratio tracers also show consistent breaks at $\lambda_{\rm Edd}\sim0.004$. 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 $\lambda_{\rm Edd}$, indicating a relative suppression of radio emission as the disk becomes more dominant with respect to the X-ray corona. Taken together, these results provide evidence that Mkn~590 is undergoing a state transition, supporting a broad analogy between changing-look AGNs and X-ray binaries.