Exploring the Accretion Disk/Corona Connection in NGC 6814: Insights from UV and X-Ray Spectral-timing Studies

Abstract We conducted a comprehensive spectral and timing analysis of NGC 6814 using AstroSat’s 2019 and XMM-Newton’s 2021 observations. A cross-correlation analysis revealed a significant correlation between far-UV (1541 Å)/X-ray and UVW1 (2910 Å)/X-ray variations, with delays of ∼15 ks and 30 ks, respectively. We constructed four broadband spectral energy distributions (SEDs) after applying aperture correction (for the Ultra-Violet Imaging Telescope filter), subtracting host galaxy and emission-line contributions from UV flux, and using mean X-ray spectra alongside selected UV data points. First, we fitted the SEDs with the kynsed model, assuming various combinations of inclination θ , color correction factors f col , and black hole spins. Best-fit models were achieved for θ = 70° (consistent with past estimates for this source) and for spin ≤0.5, while f col is not constrained. kynsed provided a satisfactory fit to all SEDs in the case when the corona is powered by the accretion process, with ∼10%–20% of the accretion power transferred to the corona, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">Edd</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>0.1</mml:mn> </mml:math> , a corona radius of ∼6–10 r g , and a height of ∼7.5–35 r g . Model time lags computed using the SED best-fit results are aligned well with the observed time lags. Although some of the model parameters are not constrained, the important result of our work is that both the broadband X-ray/UV spectra and the X-ray/UV time lags in NGC 6814 are consistent with the hypothesis of X-ray illumination of the disk in a lamp-post geometry framework. Within this model framework, we do not need to assume an outer or inner truncated disk.