Hengxiao Guo

Changing-look active galactic nuclei (CL-AGNs) provide a unique opportunity to probe the coupling between accretion flows and relativistic jets in supermassive black holes. We investigate the long-term radio behavior of CL-AGNs over ∼20 yr by combining FIRST and VLASS observations with quasi-simultaneous optical spectroscopy and photometry. From a parent sample of 1092 CL-AGNs, we identify 58 sources with radio detections. Radio-detected CL-AGNs exhibit systematically higher radio kinetic efficiency, quantified by P _j / L _bol , than both typical radio-detected AGNs and radio transients, consistent with their preference for low Eddington ratios. At the population level, the expected anticorrelation between radio emission and accretion rate is weak. However, a clear source-by-source anticorrelation emerges in a small subset of CL-AGNs with continuous multiepoch coverage. We further identify four radio transients, including both radio turn-on and turn-off events, and one source exhibiting a multiwavelength flare that may be indicative of tidal disruption event-like activity. These results suggest that radio activity in CL-AGNs is not governed by instantaneous accretion state changes but is instead regulated by long-term accretion history and jet evolution, with additional stochastic or transient channels contributing in rare cases.

We present the most densely sampled multiband optical photometric observations of the peculiar BL Lacertae object OJ 287 from 2015 to 2025 with a focus on its optical activity on diverse timescales. We present a total of 2296, 10,927, 11,484, and 2982 data points in B , V , R , and I bands, respectively. The densely sampled observations allow us to keep track of the source evolution that it has exhibited since the start of the predicted major optical flaring activity at the end of 2015. The study reveals clear and persistent bluer-when-brighter trends in both the long-term and short-term variations. Different bands were cross-correlated with discrete correlation functions, which peak at zero lag, implying cospatial emission. Using eight optical spectra in the low-flux states of OJ 287 taken from 2017 October 21 to 2017 November 22, from Steward Observatory, we estimate the central black hole mass to be at least 3.89 × 10 ⁹ M _⊙ from the [O III ] line width. The emission mechanism of the binary black hole blazar, and its possible implication in various aspects of multimessenger astronomy, are briefly discussed.

Accretion disks in active galactic nuclei (AGN) are promising sites for mergers of stellar-mass black holes (BHs) detectable via gravitational waves (GWs). These environments facilitate both in situ formation and dynamical capture of compact objects and their subsequent mergers. The uncertain origin of GW events detected by LIGO, Virgo, and KAGRA motivates searching for accompanying electromagnetic (EM) signatures. Here, we investigate postmerger EM flares associated with jets launched from merger remnants, as well as from the shocked ambient gas as the jet breaks out of the disk. We find that jet breakout produces luminous gamma-ray emission, detectable with MeV-band telescopes. Cooling emission from a shocked circum-BH minidisk, winds, and background AGN disk peaks in the UV and optical, with durations ranging from about an hour to a month, and can be identified through year-long monitoring of ∼10 ³ AGNs with luminosities ranging from ∼10 ⁴⁴ to ∼10 ⁴⁵ erg s ⁻¹ . With a single set of parameters, this postmerger jet model produces gamma-ray, hard X-ray, and optical flares similar to those claimed to be associated with GW events. Furthermore, by incorporating a transition from a high- to low-angular-momentum accretion state after the merger, the model avoids excessive BH growth, alleviating tensions with hyper-Eddington accretion scenarios.

NGC 4395 is a nearby dwarf spiral galaxy hosting an active galactic nucleus (AGN) powered by an intermediate-mass black hole (IMBH; M _BH ∼ 10 ⁴⁻⁵ M _⊙ ). Recent optical continuum reverberation mapping studies have suggested potential lag variations between different epochs, offering important clues to the physical mechanisms governing variability in the vicinity of the central black hole. We present continuous intranight multiband photometric monitoring of NGC 4395 based on five nights of observations, including three nights from the Faulkes Telescope North (two archival) and two new nights from Mephisto. This represents the first systematic investigation of optical continuum lag stability in a robustly confirmed IMBH. By applying difference-imaging techniques to both the new observations and the reprocessed archival data, we significantly detect optical interband lags of ∼5–15 minutes, which increase monotonically with wavelength. No obvious u -band lag excess is observed, implying a negligible fractional contribution from diffuse continuum (DC) emission to the optical continuum, in agreement with our spectral decomposition results. Remarkably, the interband lags remain stable over multiyear baselines. We suggest that this long-term lag stability may be related to the minor DC contribution, a relatively steady disk–corona structure, and the unusually high X-ray-to-optical luminosity ratio characteristic of low-luminosity AGNs, which likely allows X-ray reprocessing to dominate over other potential variability mechanisms. Future facilities like Gemini/SCORPIO, with its simultaneous optical to near-infrared coverage, will be ideally suited to play an important role in advancing this field.

X-ray timing of active galactic nuclei (AGNs) provides a unique probe of gas accretion onto supermassive black holes (SMBHs). Quasi-periodic oscillations (QPOs), which trace gas dynamics in the strongly curved spacetime around SMBHs, are rare in AGN. These signals are often analogs of high-frequency QPOs occasionally seen in some black-hole X-ray binaries, and their scarcity in AGN can partly be attributed to the low frequencies expected for typical SMBH masses. Intriguingly, robust X-ray QPO detections in SMBH systems have so far been reported only in narrow-line Seyfert 1 galaxies (NLS1s) and tidal disruption events (TDEs). Here we report the discovery of a QPO candidate during the 2018 outburst of the changing-look AGN (CL-AGN) NGC 1566. Numerical simulations indicate that the disk epicyclic oscillations responsible for high-frequency QPOs are damped by magnetohydrodynamic turbulence unless the accretion flow is misaligned and/or eccentric. In TDEs, the stellar debris stream is naturally misaligned with the SMBH spin, while NLS1s may host misaligned disks due to their youth. Motivated by the QPO candidate in NGC 1566, we propose that CL-AGN accretion is also misaligned – potentially fueled by captured, free-falling broad-line region clouds. This model naturally explains why CL-AGN transition timescales are much shorter than the standard disk viscous timescale. This picture can be tested by searching for QPOs or quasi-periodic eruptions in other CL-AGNs.

X-ray quasiperiodic eruptions (QPEs) represent a novel population of extreme, repeating nuclear transients whose physical origins remain debated. A defining characteristic of QPEs has been their exclusive detection in the X-ray band, with a notable absence of correlated multiwavelength counterparts. Here we report the first detection of a recurrent ultraviolet (UV) response temporally coupled to the X-ray QPE signal in the source Ansky/ZTF19acnskyy. The UV emission displays coherent periodic modulations over five consecutive cycles, systematically lagging the X-ray eruptions by 0.9 6 − 0.39 + 0.38 days, with a cross-correlation coefficient of r max ∼ 0.6 . We suggest that the detectability of this corresponding signal may be enabled by Ansky’s unusually long recurrence timescale, which could reduce the temporal smearing of the UV response seen in more rapid QPEs. The observed delay may correspond to a diffusion timescale associated with heated blobs. However, we cannot exclude the possibility that the lag corresponds to the light-crossing time associated with X-ray irradiation that originates near the central black hole and propagates to the outer UV-emitting region. While numerous QPE models have been proposed, any viable model for Ansky must be able to simultaneously explain the presence of a UV counterpart, its measured time lag, and the previously observed steadily increasing recurrence period.

Recent reverberation delay measurements have moved beyond the 10 keV X-ray range, providing evidence for the Compton hump (aka the reflection hump) in the lag spectra. We report the relativistic reverberation of the reflection hump in the bright Seyfert 1 galaxy IC 4329A based on a long Nuclear Spectroscopic Telescope Array (NuSTAR) observation. We find a delayed response of the 20–30 keV X-ray band, with a lag time of ∼1825 s at frequencies of 0.5–1.5 × 10 ⁻⁴ Hz. The lag amplitude drops to ∼195 s as the frequencies increase to (1.5–10) × 10 ⁻⁴ Hz. Including IC 4329A, so far five sources have been explored for reflection hump reverberation. We perform reverberation modeling of the 3–50 keV lag-energy spectra using the general relativistic transfer function code, which provides independent timing-based measurements of the black hole mass M BH = 1.3 7 − 0.36 + 0.33 × 1 0 8 M ⊙ and the coronal height h = 2.4 5 − 2.36 + 1.92 R g (with uncertainties at 90% confidence). Within the uncertainties, the measured mass is found to be consistent with the previous finding. Furthermore, we undertake reflection spectroscopy to account for the hump feature and the associated relativistic effect using the time-averaged flux spectrum. Further sampling of the NuSTAR data (with a bin width of 0.2/0.4 keV below and above 10 keV) that reshapes the spectral resolution allows us to constrain the coronal temperature at 50.2 6 − 4.03 + 5.58 keV—consistent with the previous result from the combined Suzaku and NuSTAR data.

Interband lags in the optical continua of active galactic nuclei (AGNs) have been observed over years of monitoring, yet their physical origins remain unclear. While variable interband lags have potentially been found in a few individual AGNs, the temporal behavior of the interband lags of an AGN sample has not been explored systematically. Here, we analyze the interband lags of 94 bright AGNs at z < 0.8, using both seasonal 1 yr and full 6 yr gri-band light curves from Zwicky Transient Facility Data Release 22. We find that more than half of the 94 AGNs show significant seasonal variations in the interband lags. Besides, the short-term lags, derived by averaging the lags inferred from multiple seasonal light curves, are consistently smaller than the long-term lags, which are inferred from the full 6 yr light curves. This supports recent theoretical simulations where the lag measurement is sensitive to the baseline of the light curve and the lag variation could be simply attributed to the inherent randomness of AGN variability. Our findings suggest that the interband lags of AGNs are more complex and stochastic than commonly thought, and highlight the importance of high-precision time-domain surveys for uncovering the properties of AGN variability as well as the associated accretion physics.