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Zheng-Yan Liu

Publications and source records attributed to Zheng-Yan Liu.

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Exploring Hierarchical Merger Scenarios for GW241011 and GW241110

GW241011 and GW241110 are asymmetric binary black hole mergers with rapidly spinning primaries, unequal component masses, and nonzero spin--orbit tilts, making them natural candidates for hierarchical mergers. We use a Bayesian framework to compare a fiducial first-generation (1G+1G) binary black hole population with second-generation plus first-generation (2G+1G) hierarchical merger models in star clusters and active galactic nucleus (AGN) disks. Both events favor the 2G+1G interpretation over the 1G+1G hypothesis, with $\ln\mathcal{B}^{\rm 2G+1G}_{\rm 1G+1G}\simeq6.5$--$8.6$ for GW241011 and $\ln\mathcal{B}^{\rm 2G+1G}_{\rm 1G+1G}\simeq3.0$--$4.5$ for GW241110, depending on the waveform model and assumed environment. The AGN disk models yields slightly larger evidence than the star cluster models, mainly due to their spin tilt distribution, but the data do not provide a decisive environmental classification. We further consider a third-generation plus first-generation (3G+1G) interpretation, but it is not robustly preferred over 2G+1G scenarios. Finally, we also search for optical counterparts by examining AGNs within the three-dimensional localization volumes using ZTF and ATLAS forced photometry, and find one candidate source with weak flare, which might be associated with GW241110 event.

astro-ph.HE

Searching for Electromagnetic Counterpart Candidates to GW231123

The detection of GW231123, a gravitational-wave (GW) event with exceptionally massive and rapidly spinning black holes, suggests the possible formation within an active galactic nucleus (AGN) disk, which provides a favorable environment for potentially generating an observable electromagnetic (EM) counterpart. We conduct a search for such a counterpart by crossmatching the GW localization with a comprehensive catalog of AGN flares from the Zwicky Transient Facility. Our analysis yields six plausible optical flare candidates that are spatially and temporally coincident with GW231123 and exhibit significant deviations from their AGN baseline flux. Although these candidates represent a crucial first step, their true nature remains inconclusive. Confirming any one of these flares via future observations would provide a landmark validation of the AGN formation channel and unlock the multi-messenger potential of this extraordinary merger.

astro-ph.HE

A Systematic Search for Active Galactic Nucleus Flares in ZTF Data Release 23

Active galactic nuclei (AGNs) are known to exhibit stochastic variability across a wide range of timescales and wavelengths. AGN flares are extreme outbursts that deviate from this typical behavior and may trace a range of energetic physical processes. Using six years of data from Zwicky Transient Facility (ZTF) Data Release 23, we conduct a systematic search for AGN flares among a sample of well-sampled AGNs and AGN candidates. We construct two catalogs: the AGN Flare Coarse Catalog (AGNFCC), containing 28,504 flares identified via Bayesian blocks and Gaussian Processes, and the AGN Flare Refined Catalog (AGNFRC), comprising 1,984 high-confidence flares selected using stricter criteria. We analyze their spatial distribution, temporal characteristics, host AGN type and potential origins. Some flares can be associated with known supernovae, tidal disruption events, or blazars, and a few may be linked to binary black hole mergers or microlensing events. These catalogs provide a valuable resource for studying transient phenomena in AGNs and are publicly available at https://github.com/Lyle0831/AGN-Flares.

astro-ph.HE

LVK S241125n: Massive Binary Black Hole Merger Produces Gamma Ray Burst in Active Galactic Nucleus Disk

Recently, the gravitational-wave (GW) event S241125n, detected by LIGO/Virgo/KAGRA (LVK), has been reported to coincide with a candidate detected by Swift-BAT/GUANO and an X-ray candidate found by FXT onboard of Einstein Probe (EP) and confirmed by Swift-XRT. We estimate that the joint false alarm rate (FAR) for the three candidates is 1 / 30 yr and that the corresponding false alarm probability (FAP) is $\mathrm{FAP}_{\rm triple} = 0.037$ ($1.8 σ$). The coincidence between the GW and GRB could be an interesting test of their origin and open attractive opportunities for multi-messenger observations, if they are actually associated. Motivated by this, we propose a theoretical model in which a binary black hole (BBH) merger occurs within an active galactic nucleus (AGN) disk. The typically massive and significantly kicked merger remnant accretes disk material at hyper-Eddington rates, and the resulting jet could lead to the GRB associated with the GW event. As the jet interacts with the gas in the AGN disk, the shock breakout produces a Comptonized spectrum, consistent with an unusually soft photon index of the GRB prompt emission observed by Swift-BAT following LVK S241125n. Meanwhile, strong absorption and dust extinction of the afterglow by the high column density typical of AGN disks could explain the unusually hard spectrum observed in the X-ray band by EP, as well as the non-detection of an optical counterpart. Our model is predictive, and we highlight the importance of further constraining the orbital eccentricity of the merger and conducting deep-field observations of the host galaxy to test our explanation.

astro-ph.HE

Target of Opportunity Observations Detectability of Kilonovae with WFST

Kilonovae are approximately thermal transients, produced by mergers of binary neutron stars (BNSs) and NS-black hole binaries. As the optical counterpart of the gravitational wave event GW170817, AT2017gfo is the first kilonova detected with smoking-gun evidence. Its observation offers vital information for constraining the Hubble constant, the source of cosmic $r$-process enrichment, and the equation of state of neutron stars. The 2.5-meter Wide-Field Survey Telescope (WFST) operates at six bands (u, g, r, i, z, w), spanning from 320 to 925 nm. It will be completed in the first half of 2023, and with a field-of-view diameter of 3 degrees, aims to detect kilonovae in the near future. In this article, considering the influence of the host galaxies and sky brightness, we generate simulated images to investigate WFST's ability to detect AT2017gfo-like kilonovae. Due to their spectra, host galaxies can significantly impact kilonova detection at a longer wavelength. When kilonovae are at peak luminosity, we find that WFST performs better in the g and r bands and can detect 90\% (50\%) kilonovae at a luminosity distance of 248 Mpc (338 Mpc) with 30 s exposures. Furthermore, to reflect actual efficiency under target-of-opportunity observations, we calculate the total time of follow-up under various localization areas and distances. We find that if the localization areas of most BNS events detected during the fourth observing (O4) run of LIGO and Virgo are hundreds of deg$^2$, WFST is expected to find $\sim$30\% kilonovae in the first two nights during O4 period.

astro-ph.HE