Characterizing X-ray, UV, and optical variability in NGC 6814 using high-cadence Swift observations from a 2022 monitoring campaign

Abstract

ABSTRACT We present the first results of a high-cadence Swift monitoring campaign (3–4 visits per day for 75 d) of the type 1.5 Seyfert galaxy NGC 6814 characterizing its variability throughout the X-ray and ultraviolet (UV)/optical wavebands. Structure function analysis reveals an X-ray power law ($\alpha =0.5^{+0.2}_{-0.1}$) that is significantly flatter than the one measured in the UV/optical bands (〈α〉 ≈ 1.5), suggesting different physical mechanisms driving the observed variability in each emission region. The structure function break-time is consistent across the UV/optical bands (〈τ〉 ≈ 2.3 d), suggesting a very compact emission region in the disc. Correlated short time-scale variability measured through cross-correlation analysis finds a lag–wavelength spectrum that is inconsistent with a standard disc reprocessing scenario (τ ∝ λ4/3) due to significant flattening in the optical wavebands. Flux–flux analysis finds an extremely blue active galactic nucleus (AGN) spectral component (Fν ∝ λ−0.85) that does not follow a standard accretion disc profile (Fν ∝ λ−1/3). While extreme outer disc truncation (Rout = 202 ± 5 rg) at a standard accretion rate ($\dot{m}_{\mathrm{Edd}}=0.0255\pm 0.0006$) may explain the shape of the AGN spectral component, the lag–wavelength spectrum requires more modest truncation ($R_{\mathrm{out}}=1382^{+398}_{-404}\,r_g$) at an extreme accretion rate ($\dot{m}_{\mathrm{Edd}}=1.3^{+2.1}_{-0.9}$). No combination of parameters can simultaneously explain both results in a self-consistent way. Our results point towards the possibility of a non-standard disc geometry in NGC 6814.