Reverberation mapping of active galactic nuclei: from X-ray corona to dusty torus.

We present the first results from the ongoing, intensive, multi-wavelength
monitoring program of the luminous Seyfert 1 galaxy Mrk 817. While this AGN
was, in part, selected for its historically unobscured nature, we discovered
that the X-ray spectrum is highly absorbed, and there are new blueshifted,
broad and narrow UV absorption lines, which suggest that a dust-free, ionized
obscurer located at the inner broad line region partially covers the central
source. Despite the obscuration, we measure UV and optical continuum
reverberation lags consistent with a centrally illuminated Shakura-Sunyaev thin
accretion disk, and measure reverberation lags associated with the optical
broad line region, as expected. However, in the first 55 days of the campaign,
when the obscuration was becoming most extreme, we observe a de-coupling of the
UV continuum and the UV broad emission line variability. The correlation
recovers in the next 42 days of the campaign, as Mrk 817 enters a less obscured
state. The short CIV and Ly alpha lags suggest that the accretion disk extends
beyond the UV broad line region.

AGN STORM 2. I. First results: A Change in the Weather of Mrk 817

We present a new method of discovering galaxy-scale, strongly-lensed QSO systems from unresolved light curves using the autocorrelation function. The method is tested on five rungs of simulated light curves from the Time Delay Challenge 1 that were designed to match the light-curve qualities from existing, ongoing, and forthcoming time-domain surveys such as the Medium Deep Survey of the Panoramic Survey Telescope And Rapid Response System 1, the Zwicky Transient Facility, and the Rubin Observatory Legacy Survey of Space and Time. Among simulated lens systems for which time delays can be successfully measured by current best algorithms, our method achieves an overall true positive rate of 28--58% for doubly-imaged QSOs (doubles) and 36--60% for quadruply-imaged QSOs (quads) while maintains $\lesssim$10% false positive rates. We also apply the method to observed light curves of 22 known strongly-lensed QSOs, and recover 20% of doubles and 25% of quads. The tests demonstrate the capability of our method for discovering strongly-lensed QSOs from major time domain surveys. The performance of our method can be further improved by analysing multi-filter light curves and supplementing with morphological, colour, and/or astrometric constraints. More importantly, our method is particularly useful for discovering small-separation strongly-lensed QSOs, complementary to traditional imaging-based methods.

Discovering strongly lensed QSOs from unresolved light curves


 We have measured the wavelength-dependent lags between the X-ray, UV and optical bands in the high accretion rate ($L/L_{\rm Edd}\approx 40{{\ \rm per\ cent}}$) Active Galactic Nucleus Mrk 110 during two intensive monitoring campaigns in February and September 2019. We divide the observations into three intervals with different X-ray luminosities. The first interval, already published in Vincentelli et al. (2021), has the lowest X-ray luminosity and did not exhibit the U-band excess positive lag, or the X-ray excess negative lag that is seen in most AGN. However, these excess lags are seen in the two subsequent intervals of higher X-ray luminosity. Although the data are limited, the excess lags appear to scale with X-ray luminosity. Our modelling shows that lags expected from reprocessing of X-rays by the accretion disc vary hardly at all with increasing luminosity. Therefore, as the U-band excess almost certainly arises from Balmer continuum emission from the broad line region (BLR), we attribute these lag changes to changes in the contribution from the BLR. The change is easily explained by the usual increase in the inner radius of the BLR with increasing ionising luminosity.

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

Jet launching in radio loud (RL) quasars is one of the fundamental problems in astrophysics. Exploring the differences in the inner accretion disk properties between RL and radio quiet (RQ) quasars might yield helpful clues to this puzzle. We previously discovered that the shorter term UV/optical variations of quasars are bluer than the longer term ones, i.e., the so-called timescale-dependent color variation. This is consistent with the scheme that the faster variations come from the inner and hotter disk regions, thus providing a useful tool to map the accretion disk which is otherwise unresolvable. In this work we compare the UV/optical variations of RL quasars in SDSS Stripe 82 to those of several RQ samples, including those matched in redshift-luminosity-black hole mass and/or color-magnitude. We find that while both RL and RQ populations appear bluer when they brighten, RL quasars potentially show a weaker/flatter dependence on timescale in their color variation. We further find that while both RL and RQ populations on average show similar variation amplitudes at long timescales, fast variations of RL sources appear weaker/smaller (at timescales of ~ 25 -- 300 days in the observer's frame), and the difference is more prominent in the g-band than in the r-band. Inhomogeneous disk simulations can qualitatively reproduce these observed differences if the inner accretion disk of RL quasars fluctuates less based on simple toy models. Though the implications are likely model dependent, the discovery points to an interesting diagram that magnetic fields in RL quasars may be prospectively stronger and play a key role in both jet launching and the stabilization of the inner accretion disk.

New Clues to Jet Launching: The inner disks in radio loud quasars may be more stable

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 $\sim 3$) 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 UV/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, $\tau_{\mathrm{TH}}\propto L^{0.5}$. 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.

Faint AGNs Favor Unexpectedly Long Inter-band Time Lags

The rest-frame UV/optical variability of the quasars in the Sloan Digital Sky Survey (SDSS) Stripe 82 is used to test the Corona-Heated Accretion-disk Reprocessing (CHAR) model of Sun et al. 2020. We adopt our CHAR model and the observed black-hole masses ($M_{\mathrm{BH}}$) and luminosities ($L$) to generate mock light curves that share the same measurement noise and sampling as the real observations. Without any fine-tuning, our CHAR model can satisfactorily reproduce the observed ensemble structure functions for different $M_{\mathrm{BH}}$, $L$, and rest-frame wavelengths. Our analyses reveal that a luminosity-dependent bolometric correction is disfavored over the constant bolometric correction for UV/optical luminosities. Our work demonstrates the possibility of extracting quasar properties (e.g., the bolometric correction or the dimensionless viscosity parameter) by comparing the physical CHAR model with quasar light curves.

Haikun Li

Papers

Faint AGNs Favor Unexpectedly Long Inter-band Time Lags

Modeling quasar UV/Optical variability with the corona-heated accretion-disk reprocessing (CHAR) model

Modeling Quasar UV/Optical Variability with the Corona-heated Accretion-disk Reprocessing (CHAR) Model