Hybrid corona and transient soft X-ray lags in Fairall 9

ABSTRACT Fairall 9 is among the most massive Seyfert galaxies exhibiting a strong soft X-ray excess, but it is challenging to probe soft X-ray reverberation lags (if any) due to the long intrinsic time-scales expected from its large black hole mass of $\sim 2.55 \times 10^8 \, \mathrm{M}_\odot$. We fit five XMM–Newton spectra of Fairall 9 using the hybrid reXcor model taking into account both hot and warm corona. The soft excess is explained by a combination of a physically motivated warm corona and the disc reflection. Then, we perform a wavelet coherence analysis of the light curves between 0.3–1 and 1–4 keV bands. The spectral fits are consistent with a rapidly spinning black hole ($a = 0.99$), a warm corona with optical depth $\sim$10–30, and a hot lamp-post corona located at either 5 or $20~r_{\rm g}$. This configuration supports a coexisting hot and warm corona scenario, allowing the disc to extend almost to the event horizon. Our wavelet analysis on combined observations reveals signatures of transient soft X-ray lags, confined to specific time–frequency intervals. The earlier observations exhibit more variable and transient lag behaviour. In contrast, the later observations display more persistent soft X-ray lags at the frequencies of $\sim 9\times 10^{-6}$–$2.5 \times 10^{-5}$ Hz, with amplitudes reaching $\sim$1000 s. The results indicate a progressively stable disc–corona configuration in later observations. Given the mass and geometry of Fairall 9, the observed soft lags appears plausibly consistent in both size and time-scales with expectations from X-ray reverberation.