Yan-Rong Li

We report a rapid optical changing-look event in the Seyfert galaxy NGC 3227: the optical continuum faded by ∼70%, and broad H β emission nearly disappeared within weeks before recovering within 2 months. Damped random walk simulations calibrated to the source’s normal variability show that such a decline is unlikely for stochastic accretion fluctuations. The optical decline coincides with an X-ray spectral hardening event detected by Swift, indicating additional line-of-sight absorption. During the dim state, reverberation mapping yields a clear 6–8 day lag between the continuum and broad H β , placing the obscurer interior to the broad-line region. Photoionization calculations require a fully ionized Compton-thick column with N _H ≈ 1.5 × 10 ²⁴ cm ⁻² to reproduce the optical continuum suppression. Together, these observations demonstrate that compact, high-column-density obscurers interior to the broad-line region can produce extreme optical changing-look behavior on timescales of weeks.

Circumnuclear structures in active galactic nuclei (AGN) respond to accretion variability over different spatial and temporal scales, but whether compact narrow-line gas responds on decade timescales has not been systematically established. We use repeat Sloan Digital Sky Survey and Dark Energy Spectroscopic Instrument spectroscopy of 473 variability-selected galactic nuclei with evidence for recent nuclear brightening. We identify 19 sources whose narrow [O III ] λ 5007 flux increased by more than a factor of 2 over rest-frame baselines of 12.0–20.4 yr, which we term narrow-line ionization echoes (NIEs). In equal-weight stacked spectra, [O III ] increased by 229% ± 22%, while lower-ionization lines remained nearly unchanged or declined, indicating an ionization-dependent response rather than uniform spectral rescaling. The NIE hosts also moved coherently toward higher excitation in optical diagnostic diagrams, with mean increases of 0.46 ± 0.18 dex in log ( [ O III ] / H β ) and 0.58 ± 0.13 dex in log ( [ O III ] / [ O II ] ) . Photoionization models reproduce this evolution with a higher ionization parameter and a harder AGN-like ionizing spectrum at fixed gas abundance. For 13 sources with well-defined mid-infrared (MIR) outbursts, the integrated W1+W2 excess energy correlates strongly with the absolute [O III ] luminosity change ( r = 0.87), linking the dust and narrow-line responses to the same nuclear event. The MIR timescale and spectroscopic baseline constrain the variable [O III ]-emitting gas to characteristic scales of 1.5–5.2 pc. Compact narrow-line gas therefore appears to form an intermediate, decade-scale response layer between the infrared dust echo and the long-lived extended narrow-line region.

Cross correlation is crucial in studies of multiwavelength flux variability in active galactic nuclei (AGNs), especially for reverberation mapping analysis, where an interpolated cross-correlation function is widely used to measure time lags between light curves. While time-lag uncertainties can be estimated via the flux randomization and random subset selection method, an appropriate framework for assessing cross-correlation significance remains lacking in the literature. Here, we attempt to fill this gap by leveraging the well-established property from stochastic time series theory, namely, that the probability distribution of cross-correlation coefficients for independent stochastic light curves asymptotically approaches a normal distribution. Its variance can be analytically estimated using the autocorrelation functions of the light curves. We employ Monte Carlo simulations to validate this property for irregularly sampled, red-noise AGN light curves and then propose a fast procedure to perform null-hypothesis testing for the cross correlation of AGN light curves. We also present exemplary applications to AGN reverberation mapping data. This procedure requires prior determination of the autocorrelation functions of light curves, which can be obtained via model fitting. The long-standing issue regarding unbiasedly recovering the autocorrelation function remains unresolved when light-curve duration is comparable to the typical variation timescale, warranting further future investigation.

We present the ground-based imaging campaign and light curves of Markarian 817 as part of the multiwavelength monitoring program AGN STORM 2. Observations were carried out over 1.4 yr in the uBgVriz filters, with a median cadence of 0.4 day in the g band. Reverberation lags are measured using three methods (interpolated cross-correlation function (ICCF), Just Another Vehicle for Estimating Lags In Nuclei, and PyROA ) with the Swift UVW2 band (1928 Å) as the reference light curve. The ICCF centroid lags range from 3.0 ± 0.8 days for the u band up to 7.9 ± 1.5 days for z , and are consistent with a τ ∝ λ ^4/3 dependence, the relation expected for lamppost reprocessing by a Shakura–Sunyaev disk. Lags measured with the other methods are systematically shorter, and deviate from a λ ^4/3 power-law spectrum at long wavelengths. The lags exceed thin-disk reprocessing predictions by factors of ∼3–6, similar to the “disk size discrepancy” seen in other Seyfert galaxies. We divide the campaign into three epochs with different levels of mean luminosity and X-ray obscuring column density and find that the lags vary by as much as a factor of 2 between epochs. The intrinsic spectral energy distribution is bluer and brighter during the first third of the campaign, and the longest continuum reverberation lags are obtained during that period. These results suggest that changes in ionizing luminosity can produce large variations in continuum lags on short timescales by altering the diffuse continuum luminosity emitted by the broad-line region (BLR) and/or obscuring outflow, although changes in obscuration between the central engine and BLR may also contribute to the lag variations.

As the 15th paper in a series reporting on a large reverberation mapping (RM) campaign of super-Eddington accreting massive black holes (SEAMBHs) in active galactic nuclei (AGNs), we present the results of measurements of the Mg ii lines in 18 SEAMBHs monitored spectroscopically from 2017 to 2024. Among these, the time lags of Mg ii have been successfully determined for eight of the 18 objects, thereby expanding the current Mg ii RM sample, particularly at higher accretion rates. By incorporating measurements of the line widths, we determine the masses of their central supermassive black holes. Based on these new measurements, we update the relation between the Mg ii radius and the monochromatic luminosity at 3000 Å ( R _MgII – L ₃₀₀₀ relation), yielding a slope of 0.24 ± 0.03, which is slightly shallower than, yet still consistent with, previously reported values. Similar to the H β lines, the Mg ii time lags in SEAMBHs are shorter than those of AGNs with normal accretion rates at comparable luminosities. The deviation of AGNs from the best-fit R _MgII – L ₃₀₀₀ relation shows a strong correlation with the accretion rate, while no significant correlation is found between the deviation and the flux ratio of UV iron to Mg ii .

JWST/NIRCam observations reveal dense stellar cores in high-redshift galactic bulges, indicative of sustained star formation and potential stellar accretion. We introduce accretion-modified stars (AMSs) as a new component in the chemical evolution of high-redshift bulges hosting active galactic nuclei (AGNs). The gas-phase chemical evolution of bulge environments containing AMSs is modeled within 1 Gyr by combining formalisms for population evolution and galactic chemical evolution, and observational signatures are tracked via photoionization modeling on Baldwin–Phillips–Terlevich (BPT) diagrams. Sustained high accretion onto AMSs leads to rapid gas-phase metal enrichment of the bulge, producing abundance peaks up to five times solar metallicity within 0.1 Gyr and significantly modifying elemental ratios in the gas phase. Atypical gas-phase abundance patterns occur during early high-accretion phases and gradually diminish as the accretion rate declines. In BPT diagrams, high-AMS-accretion scenarios shift the modeled emission-line sequence toward the local AGN branch and extend into the high-metallicity regime. Supersolar narrow-line regions observed in AGNs at z ≳ 15 may reflect such AMS-driven gas-phase enrichment of the host bulge under extreme gas densities. While direct detection of AMSs within AGN bulges remains challenging, the model provides testable predictions for future spectroscopic surveys and motivates further exploration of noncanonical stellar populations in AGN host bulges.

The AGN Space Telescope and Optical Reverberation Mapping (STORM) 2 campaign targeted Mrk 817 with intensive multiwavelength monitoring and found its soft X-ray emission to be strongly absorbed. We present results from 157 near-IR spectra with an average cadence of a few days. Whereas the hot dust reverberation signal as tracked by the continuum flux does not have a clear response, we recover a dust reverberation radius of ∼90 lt-days from the blackbody dust temperature light curve. This radius is consistent with previous photometric reverberation mapping results when Mrk 817 was in an unobscured state. The heating/cooling process we observe indicates that the inner limit of the dusty torus is set by a process other than sublimation, rendering it a luminosity-invariant “dusty wall” of a carbonaceous composition. Assuming thermal equilibrium for dust optically thick to the incident radiation, we derive a luminosity of ∼6 × 10 ⁴⁴ erg s ⁻¹ for the source heating it. This luminosity is similar to that of the obscured spectral energy distribution, assuming a disk with an Eddington accretion rate of m ̇ ∼ 0.2 . Alternatively, the dust is illuminated by an unobscured lower luminosity disk with m ̇ ∼ 0.1 , which permits the UV–optical continuum lags in the high-obscuration state to be dominated by diffuse emission from the broad-line region. Finally, we find hot dust extended on scales ≳ 140–350 pc, associated with the rotating disk of ionised gas we observe in spatially resolved [S III ] λ 9531 images. Its likely origin is in the compact bulge of the barred spiral host galaxy, where it is heated by a nuclear starburst.

From a parent catalog of 561 changing-look active galactic nuclei (CL-AGNs) identified by W.-J. Guo et al., we investigate the evolutionary sequence of broad emission lines using a redshift-selected subset (0.35 < z < 0.45) of 54 CL-AGNs whose Dark Energy Spectroscopic Instrument (DESI) spectra simultaneously cover the H α , H β , and Mg ii emission lines. To provide a baseline for comparison, we construct a control sample of 19,897 normal Type 1 AGNs within the same redshift range from the DESI Year 1 data. Through stacked spectral analysis and line-continuum luminosity correlations, we identify a clear evolutionary sequence in all AGNs where broad H β fades first, followed by Mg ii, and then H α , as the AGN luminosity declines—consistent with expectations from reverberation mapping. This trend reflects a radially stratified broad-line region (BLR), where each line’s responsivity depends on its ionization potential and radial distance from the central engine. In addition, we find that more massive supermassive black holes require lower Eddington ratios to fully suppress broad emission lines, suggesting that the critical accretion threshold for the changing-look phenomenon is mass-dependent. Our results present the first statistical confirmation of a stratified broad-line fading sequence in AGNs, reinforcing the central role of accretion state in shaping BLR structure and visibility.

Robust extragalactic distance measurements are crucial for resolving the persistent discrepancy in the value of the Hubble constant ( H ₀ ). Active galactic nuclei (AGNs), through their compact and variable broad-line regions, enable the determination of geometric distances when reverberation mapping (RM) is combined with spectroastrometry. We report results from a spectroscopic RM campaign (2022 October–2023 March) targeting two narrow-line Seyfert 1 galaxies, Mrk 1048 and Mrk 618, using the 3.6 m Devasthal Optical Telescope and 2 m Himalayan Chandra Telescope. High-cadence spectrophotometric monitoring was carried out using onboard instruments such as ADFOSC, HFOSC, and TANSPEC, resulting in well-sampled continuum and emission-line light curves. The observed fractional variability ( F _var ) ranged from 4% to 14% across the g -band, H β , and H α light curves. The time lags were measured using the interpolated cross-correlation function (ICCF), PyI ² CCF, and JAVELIN methods. In the rest frame, the ICCF analysis yields H β lags of 10 . 5 − 4.2 + 2.6 days for Mrk 1048 and 10 . 2 − 2.9 + 3.4 days for Mrk 618, while the corresponding H α lags are 18 . 7 − 5.4 + 5.3 and 14 . 4 − 10.5 + 4.6 days, respectively. The emission-line widths, measured from the rms spectra using σ _line , give virial black hole mass estimates of 6 . 3 − 2.1 + 2.0 × 1 0 7 M ⊙ for Mrk 1048 and 1 . 2 − 0.6 + 0.4 × 1 0 7 M ⊙ for Mrk 618. These results will serve as a basis for absolute geometric distance calibration when combined with VLTI/GRAVITY spectroastrometric measurements, thereby contributing to the development of AGNs as standardizable cosmological probes.