Faint Active Galactic Nuclei Favor Unexpectedly Long Inter-band Time Lags

Abstract

Inconsistent conclusions are obtained from recent active galactic nuclei (AGNs) accretion disk inter-band time-lag measurements. While some works show that the measured time lags are significantly larger (by a factor of ∼3) than the theoretical predictions of the Shakura & Sunyaev disk (SSD) model, others find that the time-lag measurements are consistent with (or only slightly larger than) that of the SSD model. These conflicting observational results might be symptoms of our poor understanding of AGN accretion physics. Here we show that sources with larger-than-expected time lags tend to be less luminous AGNs. Such a dependence is unexpected if the inter-band time lags are attributed to the light-travel-time delay of the illuminating variable X-ray photons to the static SSD. If, instead, the measured inter-band lags are related not only to the static SSD but also to the outer broad emission-line regions (BLRs; e.g., the blended broad emission lines and/or diffuse continua), our result indicates that the contribution of the non-disk BLR to the observed ultraviolet/optical continuum decreases with increasing luminosity (L), i.e., an anti-correlation resembling the well-known Baldwin effect. Alternatively, we argue that the observed dependence might be a result of coherent disk thermal fluctuations as the relevant thermal timescale, τ TH ∝ L 0.5. With future accurate measurements of inter-band time lags, the above two scenarios can be distinguished by inspecting the dependence of inter-band time lags upon either the BLR components in the variable spectra or the timescales.

The Sloan Digital Sky Survey Reverberation Mapping Project: UV–Optical Accretion Disk Measurements with the Hubble Space Telescope

“…Li et al (2021) show that this magnetic-coupling model is consistent with observations of the full sample of disk lags, with lower-luminosity AGNs typically having longer lags. The luminosity distributions of our samples are broad enough that we cannot conclusively test this theory.…”

The luminosity-dependent contribution from the broad-line region to the wavelength-dependent lags in Mrk 110

“…at the Balmer Jump) is dominated by the BLR, leading to the so called "U-band excess" in almost all the observed sources (Korista & Goad 2001, 2019Cackett et al 2018;Lawther et al 2018). Moreover, the X-ray vs UVW2 lags seem to show a larger-than-expected amplitude when compared to standard accretion disc models (see also Li et al 2021). This led to the development of several models attempting to reproduce the lag spectrum with reprocessing driven in a lamp-post model with a relatively high corona (Kammoun et al 2021a), an extended reprocessor (Gardner & Done 2015), or even Vincentelli et al with the BLR (Korista & Goad 2001, 2019Lawther et al 2018;Chelouche et al 2019;Netzer 2020Netzer , 2021Cackett et al 2021a).…”

The luminosity-dependent contribution from the broad line region to the wavelength-dependent lags in Mrk 110

“…at the Balmer Jump) is dominated by the BLR, leading to the so called "U-band excess" in almost all the observed sources (Korista & Goad 2001, 2019Cackett et al 2018;Lawther et al 2018). Moreover, the X-ray vs UVW2 lags seem to show a larger-than-expected amplitude when compared to standard accretion disc models (see also Li et al 2021). This led to the development of several models attempting to reproduce the lag spectrum with reprocessing driven in a lamp-post model with a relatively high corona (Kammoun et al 2021a), an extended reproces-sor (Gardner & Done 2015), or even with the BLR (Korista & Goad 2001, 2019Lawther et al 2018;Chelouche et al 2019;Netzer 2020Netzer , 2021Cackett et al 2021a).…”