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

Abstract

We present the first results of a high-cadence Swift monitoring campaign ($3-4$ visits per day for $75$ days) of the Seyfert 1.5 galaxy NGC 6814 characterizing its variability throughout the X-ray and UV/optical wavebands. Structure function analysis reveals an X-ray power law ($\alpha =0.5_{-0.1}^{+0.2}$) that is significantly flatter than the one measured in the UV/optical bands ($⟨\alpha ⟩\approx 1.5$), suggesting different physical mechanisms driving the observed variability in each emission region. The structure function break-time is consistent across the UV/optical bands ($⟨\tau ⟩\approx 2.3\mathrm{d}$), 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 ($\tau \propto {\lambda }^{4/3}$) due to significant flattening in the optical wavebands. Flux-flux analysis finds an extremely blue AGN spectral component ($F_{\nu }\propto {\lambda }^{-0.85}$) that does not follow a standard accretion disc profile ($F_{\nu }\propto {\lambda }^{-1/3}$). While extreme outer disc truncation ($R_{\mathrm{out}}=202\pm 5r_g$) 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 ($R_{\mathrm{out}}=1,382_{-404}^{+398}{r}_g$) at an extreme accretion rate (${\dot{m}}_{\mathrm{Edd}}=1.3_{-0.9}^{+2.1}$). No combination of parameters can simultaneously explain both results in a self-consistent way. Our results offer the first evidence of a non-standard accretion disc in NGC 6814.

11 pages, 7 figures, 4 tables, submitted to MNRAS