Constraining the Fraction of LIGO/Virgo/KAGRA Binary Black Hole Merger Events Associated with Active Galactic Nucleus Flares

TL;DR

The paper addresses whether a subset of LVK binary black hole mergers originates in AGN disks by statistically testing spatial–temporal associations with ZTF AGN flares. It uses a boundary-aware, Voronoi-tessellation–based framework to link each BBH localization with local flare densities and employs a 200-day post-merger window to search for counterparts, yielding a maximum-likelihood estimate of $f_{ m flare} = 0.07_{-0.05}^{+0.25}$ (90% CL) driven mainly by GW190412. Excluding GW190412, the results are consistent with no association, with an upper limit of $f_{ m flare} < 0.17$ (90% CL). The analysis suggests a non-negligible fraction of BBH mergers could originate in AGN disks, motivating continued electromagnetic follow-up of well-localized, asymmetric mergers and future time-domain surveys to improve constraints and potential cosmological leverage.

Abstract

The formation channels of binary black hole (BBH) mergers detected by the LIGO/Virgo/KAGRA (LVK) network remain uncertain. While BBH mergers occurring inside active galactic nucleus (AGN) disks may interact with surrounding gas and generate observable optical flares. We test this scenario by quantifying the spatial and temporal correlation between BBH events in GWTC-4.0 and AGN flares identified from six years of the Zwicky Transient Facility (ZTF) DR23 data. Using 80 BBH mergers selected for adequate localization, redshift reach, observing-epoch overlap, and ZTF sky coverage, we construct a likelihood for the flare-associated fraction, $f_{\rm flare}$, that combines each event's 3D localization with a locally estimated flare number density derived from a 3D Voronoi tessellation, while explicitly accounting for survey boundaries and incomplete catalog coverage. Adopting a 200-day post-merger time window for potential counterparts, we infer $f_{\rm flare} = 0.07_{-0.05}^{+0.25}$ (90\% confidence level). This non-zero maximum-likelihood value is driven primarily by GW190412, for which a single flare candidate (J143041.67+355703.8) is consistent in both time and spatial position. Excluding GW190412 yields results consistent with no association and an upper limit of $f_{\rm flare} < 0.17$ at 90\% confidence level. The intrinsic properties of GW190412 and the characteristics of the candidate host AGN are broadly consistent with theoretical expectations for the AGN-disk formation channel, motivating continued, targeted electromagnetic follow-up of well-localized and highly asymmetric BBH mergers in current and upcoming time-domain surveys.

Figures (4)

• Figure 1: Probability density distributions of $f_{\rm flare}$ derived from the LVK GWTC-4.0 BBH events and the ZTF AGN candidate flares. The red solid and black dashed curves represent the results derived from the all BBH events and the set without GW190412, respectively. The vertical red and black dotted lines indicate the error interval and the upper limit at the 90% CL, correspondingly.
• Figure 2: Skymap of GW190412 event and candidate flares. The red contours represent the 50% and 90% CL sky localization regions, respectively. The gray, blue, and purple star points denote all flares in AGNFCC, those consistent with GW190412 in time, and those consistent in both time and spatial localization, correspondingly.
• Figure 3: The ZTF $g$- and $r$-band light curves for the J143041.67+355703.8 candidate flare associated with GW190412. The green and red points (with error bars) represent the apparent magnitudes in the $g$ and $r$ bands, respectively, while the blue points indicate the $g-r$ color.
• Figure 4: DESI spectrum of the host AGN of J143041.67+355703.8 flare. Vertical dotted lines mark the positions of characteristic AGN emission or absorption lines.