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 timescales expected from its large black hole mass of $\sim 2.55 \times 10^8 M_\odot$. 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 behavior. 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 timescales with expectations from X-ray reverberation.

Figures (11)

• Figure 1: Simulated wavelet coherence (WTC) spectra for various cases of light curves of lengths 25000, 50000, 75000, 100000, and 125000 s, each mimicking a single observation. In all cases, the soft-band light curves are delayed by 1000 s relative to the hard-band light curves. The color scale shows wavelet coherence magnitude, where warmer colors indicate higher coherence between the energy bands with 90 per cent confidence interval included with white contour line. The y-axis shows timescales, which are the inverse of temporal frequency. Arrows indicate phase lag direction: right-pointing arrows show in-phase variations, left-pointing show anti-phase, downward arrows indicate soft lags, and upward arrows show hard lags. The dashed line marks the cone of influence (COI), beyond which edge effects may compromise signal reliability.
• Figure 2: Time-resolved phase differences for individual cases (upper panel) and combined, stitched cases (middle panel) corresponding to the simulated WTC shown in Fig. \ref{['fig-coh-sim']}. The vertical solid lines in gray indicate the boundaries between individual observations. The color scale represents phase differences: negative values (green–blue–black) indicate soft lags, where the soft band lags behind the hard band, while positive values (yellow–orange–red) indicate hard lags. The bottom panels show the corresponding time-lag profiles, with color denoting lag amplitude. Regions with signal coherence below 0.7 are excluded. The wavelet phase and lag spectra of the stitched light curves successfully recover the imposed delays across the relevant frequencies, consistent with the results from individual segments. See text for more details.
• Figure 3: Wavelet lag profile as same as Fig. \ref{['fig-phase-dif-sim']} (bottom panel), but with Poisson noise included.
• Figure 4: Spectral data and residuals (black points) relative to the best-fitting model with $h=5~r_{\rm g}$. The panels, arranged from top-left to bottom-right, correspond to the XMM-Newton observations in chronological order, from the earliest (Obs. ID 0101040201) to the latest (Obs. ID 0741330101). Individual model components are shown as dotted lines in each panel. Note that the residuals are presented in the unit of $\sigma$
• Figure 5: Comparison of spectral model components from the best-fitting model with $h = 5~r_{\rm g}$. The total model is shown as solid lines, the reXcor component as dot-dashed lines, and other components ( cutoffpl, xillver, and additional zgauss) as dotted lines. Black, blue, red, green, and magenta correspond to different observations, ordered chronologically from the earliest (Obs. ID 0101040201) to the latest (Obs. ID 0741330101).