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 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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- January 2024
- DOI:
- 10.1093/mnras/stad3549
- arXiv:
- arXiv:2308.15543
- Bibcode:
- 2024MNRAS.527.5569G
- Keywords:
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- accretion;
- accretion discs;
- galaxies: active;
- galaxies: individual: NGC 6814;
- galaxies: Seyfert;
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 11 pages, 7 figures, 4 tables, submitted to MNRAS