Exploring the Accretion disc/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. Cross-correlation analysis revealed a significant correlation between FUV (1541 Å)/X-ray and UVW1 (2910 Å)/X-ray variations, with delays of $\sim 15\mathrm{ks}$ and $30\mathrm{ks}$, respectively. We constructed four broadband SEDs after applying aperture correction (for the UVIT 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 KYNSED model assuming various combinations of inclination, $\theta$, color correction factors, $f_{\mathrm{col}}$, and BH spins. Best-fit models were achieved for $\theta =70^{\circ }$ (consistent with past estimates for this source) and for spin $\le 0.5$, while $f_{\mathrm{col}}$ is not constrained. KYNSED provided satisfactory fit to all SEDs in the case when the corona is powered by the accretion process, with $\sim 10-20$% of the accretion power transferred to the corona, $\dot{m}/{\dot{m}}_{\mathrm{Edd}}\sim 0.1$, corona radius of $\sim 6-10r_g$, and height of $\sim 7.5-35r_g$. 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 disc in a lamp-post geometry framework. Within this model framework, we do not need to assume an outer or inner truncated disc.

Accepted for publication in ApJ, 20 Pages, 15 figures