Accretion dynamics and coronal geometry in Mrk 530: Insights from 24 years of X-ray monitoring

Abstract

We present a long-term broadband study of the Seyfert galaxy Mrk~530 spanning $\sim$24 yr (2001--2024). The source remains largely stable across epochs, except in 2018, when a possible quasi-periodic oscillation is observed simultaneously in the UV and X-ray bands, with characteristic timescales of $\sim$90 and $\sim$60 days, characterized by low coherence. Time-resolved spectral analysis shows that this epoch is characterized by comparable coronal cooling and compressional heating timescales, a condition conducive to oscillatory behavior in the inner accretion flow. Other epochs exhibit a clear mismatch between these timescales, and no such variability is observed. The X-ray spectral properties display significant long-term evolution. The photon index and luminosity vary systematically, while a soft excess is present only in early epochs (2001--2006) and weakens thereafter, consistent with an evolving warm corona. Physically motivated models indicate that changes in the accretion rate regulate both the spectral slope and coronal geometry, with higher disc accretion rates producing enhanced cooling, a more compact corona, and softer spectra, and lower rates yielding an expanded hot flow and harder emission. These results suggest that accretion-driven coupling between the disc and corona governs both the long-term spectral evolution and transient short-timescale variability in Mrk~530.

Figures (13)

• Figure 1: Energy-dependent fractional variability of Mrk 530 in the 2006 XMM-Newton (XMM3) observation.
• Figure 2: Plots obtained from the timing analysis of Mrk 530 in the 2018 Swift/XRT & UVOT observations. The upper panel shows the lightcurves in the UV (left) and X-ray (right) energy bands. The grey line depicts the average counts/s, and the green curve is the sine function fit. The lower left panel shows the results from the LS-periodogram along with $1\sigma$, $2\sigma$, and $3\sigma$ confidence levels estimated from simulating surrogate light curves. The contour plot represents the corresponding WWZ response.
• Figure 3: Correlation plots between the X-ray and the UV light curves of Mrk 530 in 2018. The count-count plot is presented in the top panel (a). The corresponding DCF and ZDCF plots are shown in the bottom panels (b) & (c), respectively. The likelihood of each ZDCF peak is shown in light green.
• Figure 4: The residuals obtained during the phenomenological spectral fitting of the 2006 XMM3 observation of Mrk 530.
• Figure 5: Residuals showing the variation in soft excess in Mrk 530 over the duration of 24 years of X-ray observations. All spectra are fitted with a Galactic absorbed power-law in the 2--10 keV energy range and then extended to lower energies.