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

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 $α_{ox}$ follows a pronounced 'V'-shaped dependence on Eddington ratio $λ_{Edd}$, with a break at $λ_{Edd}$ = 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.

Figures (8)

• Figure 1: Guideline multi-wavelength light curve of Mkn 590 compiled from various facilities between 1970 and 2025, also presented in 2014Denney2016koay. Details on data collection are described in Appendix \ref{['appendixA']} and listed in Tab. \ref{['tab:mrk590_continuum']}. In the case of optical/UV/radio measurements we present monochromatic fluxes, while in case of X-rays, the model dependent fluxes integrated over 2--10 keV are shown. The SXE and radio fluxes are offset vertically for clear comparison with other bands. The marker size of radio data scales with the beam size of radio facility. Three major CL events in the late 1980s, in the 2000s, and starting late 2023 are clearly depicted. The radio peak delay during the early 1990s is further discussed in Sec \ref{['sec:lab']}. the bright, intermediate, and faint activity level are shaded and defined empirically based on X-ray flux level and are intended for interpretive purposes only.
• Figure 2: Left: the variation of $\alpha_{\textsc{ox}}$ with $\lambda_{\rm Edd}$ from Swift monitoring of Mkn 590 from Dec. 10, 2013 to Sept. 28, 2025. The scatter of data exhibits a ' V'-shaped trend, with a break point indicated by the vertical red dashed line. The associated 95% confidence interval is shown as a red shaded band. Linear fits on either side of the break point are indicated by solid blue lines, while their 95% confidence region are shown in purple shading. Right panels: the same variation, but for $\alpha_{\textsc{ox}}^{(0.5)}$ shown in blue, $\alpha_{\textsc{ox}}$ in orange, and $\alpha_{\textsc{ox}}^{(4)}$ in green points. Points are color coded by point density to highlight regions of higher concentration and improve visualization of the underlying trend. In the case of $\alpha_{\textsc{ox}}^{(4)}$, a turnover was detected at $\log \lambda_{\rm Edd}= -1.67 \pm 0.04$, a value identical to that for the $\alpha_{\textsc{ox}}$ relation and is shown by vertical lines together with their confidence intervals as shaded regions. In the case of $\alpha_{\textsc{ox}}^{(0.5)}$, no such break point was found.
• Figure 3: A comparison of the observed $\alpha_{\textsc{ox}}$--$\lambda_{\rm Edd}$ relation (with $L_{\rm bol}$ computed by the SED integration method) for Mkn 590 given by open circles, with observations of several fading quasars marked by red diamonds Ruan2019ApJ...883...76R, and with simulated predictions for AGN marked by green squares Sobo2011MNRAS.413.2259S. The vertical dashed line represents the break point of the observed relation (see Fig. \ref{['fig:alpha_ox1']}), while blue lines show linear fits to the data, with shaded regions indicating 1$\sigma$ confidence intervals.
• Figure 4: The dependence of $\alpha_{\textsc{ox}}$ on the UV and hard X-ray accretion power tracers $\lambda L_{2500}/L_{\rm Edd}$ and $L_{\rm X}/L_{\rm Edd}$. The meaning of colors is the same as in the left panel of Fig. \ref{['fig:alpha_ox1']}.
• Figure 5: Top: The light curve of $\lambda_{\rm Edd}$, segmented into time slots named from A to D2, and color coded for reference to the following sub-panels below. Middle: From left to right: $\lambda_{\rm Edd}$ versus $HR$ for all Swift data, the same but binned across color coded time slots, and $\lambda_{\rm Edd}$ versus $\alpha_{\textsc{ox}}$ relation binned in the same way, respectively. Bottom: Binned $HR$ vs $\alpha_{\textsc{ox}}^{(0.5)}$ (right) and $\alpha_{\textsc{ox}}^{(4)}$ (left) relations showing similarly the coupled behavior of UV-thermal disk with warm corona, and warm with hot corona. The approximate timescales correspond to epochs A $\rightarrow$ B1, B1$\rightarrow$ B8, B8 $\rightarrow$ C, C$\rightarrow$D1 and D1$\rightarrow$D2, respectively.