Exploring the Accretion disc/Corona Connection in NGC 6814: Insights from UV and X-ray spectral-timing studies

TL;DR

This study analyzes NGC 6814's UV and X-ray spectral-timing data, revealing a disc-corona connection consistent with a lamp-post geometry, and constraining key physical parameters of the accretion process.

Contribution

It provides the first broadband SED modeling and time-lag analysis of NGC 6814, supporting a lamp-post corona model without requiring a truncated disc.

Findings

X-ray/UV time-lags are consistent with disc illumination by a corona.

Best-fit models suggest a corona powered by accretion with specific geometry.

Spectral-timing data favor a non-truncated, lamp-post accretion disc model.

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 \AA)/X-ray and UVW1 (2910 \AA)/X-ray variations, with delays of $\sim 15~\rm{ks}$ and $30~\rm{ks}$, 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, $f_{\rm col}$, and BH spins. Best-fit models were achieved for $\theta=70^{\circ}$ (consistent with past estimates for this source) and for spin $\leq 0.5$, while $f_{\rm col}$ 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, $\dot{m}/\dot{m}_{\rm Edd}\sim 0.1$, corona radius of $\sim 6-10~r_g$, and height of $\sim7.5-35~r_g$. 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 disc in a lamp-post geometry framework. Within this model framework, we do not need to assume an outer or inner truncated disc.