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

TL;DR

The paper investigates whether a binary black hole (BBH) merger occurring within an active galactic nucleus (AGN) disk can produce a gamma-ray burst (GRB) whose prompt and afterglow emissions accompany a gravitational-wave event, specifically LVK S241125n. It develops a physical model in which a merger remnant accretes at hyper-Eddington rates, launches a jet via the Blandford–Znajek mechanism, and produces a prompt emission with a Comptonized/thermal-dominated spectrum; the jet breakout and its interaction with disk material yield a characteristic time delay $t_{ m delay} \approx 11.264$ s and a breakout luminosity $L_{ m breakout} \approx 1.01\times10^{51}$ erg s$^{-1}$. The model also accounts for strong X-ray absorption and dust extinction in the AGN disk, explaining the unusually hard X-ray spectrum observed by Einstein Probe and the non-detection of an optical counterpart, while predicting observable infrared signatures with facilities like JWST. By fitting the predicted spectral and temporal features to the observed data and evaluating a joint false alarm probability of $\mathrm{FAP}_{\rm triple} = 0.037$ (FAR $\approx 1/30$ yr), the paper presents a testable framework for multi-messenger counterparts from BBH mergers in dense gas environments. The work highlights concrete observational tests—deep-field host-galaxy studies and infrared follow-ups—to confirm or refute the AGN-disk BBH merger scenario, advancing our understanding of GRB production channels and AGN disk physics.

Abstract

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.

Figures (9)

• Figure 1: GW skymap of LVK S241125n and the GRB location. The color bar of the skymap represents the relative probability density of the GW source location. The white solid line represents the $90\%$ confidence level contour, while the blue cross in the inset indicates the position of the candidate electromagnetic counterparts. The green curve indicates the Galactic plane.
• Figure 2: Prompt emission luminosity estimated by Swift, INTEGRAL, Fermi and Konus Wind satellites. The GW trigger time as the event start time ($T_0$) and time is in the rest-frame.
• Figure 3: The X-ray afterglow data point of S241125n (in red) is obtained using the observation of Einstein Probe within the energy range of 0.5-10 keV. For comparison, background data from Swift-XRT observations include 217 long GRBs (in gray) and 31 short GRBs (in black), covering 0.3-10 keV. The shaded regions correspond to the 90% of the confidence region generated from the data directly. It can be observed that S241125n is consistent with the luminosity of short GRBs. Time is in the rest-frame.
• Figure 4: The optical observations of S241125n include upper limits provided by Swift, COLIBRÍ, Lulin, GRANDMA, DDOTI, GOT, GIT and HCT telescopes.Upper panel shows the observed magnitude, and the lower panel shows the absolute magnitude. For comparison, the background data consists of optical observations covering multi-bands from 29 short GRBs (in gray) from 2024MNRAS.533.4023D. Among them, GRB 200826A (in blue), which has a similar redshift to S241125n, is highlighted. Time and magnitude are in the observer's frame. It should be noted that the reference background magnitudes have already been corrected for Galactic extinction, whereas the data for S241125n are mainly the raw observations directly taken from the GRB Coordinates Network (GCN). According to the Milky Way extinction in the direction of S241125n, $E(B-V) = 0.4678$ mag 2011ApJ...737..103S, the extinction-corrected values for S241125n will be approximately 0.5 to 2 mag brighter than those shown in the Figure, for examples, $A_{i'} = 0.927$ mag, $A_{r'} = 1.224$ mag and $A_{g'} = 1.739$ mag.
• Figure 5: The expected SED of a GRB originating from a BBH merger in AGN disks. In different panels, only the parameter shown in the corresponding panel is altered, while the remaining parameters remain consistent with the fiducial model. In this study, the fiducial parameters consist of: $M_{\rm SMBH} = 10^7 M_\odot$, $\dot{M}=0.01\dot{M}_{\rm Edd}$, $\tilde{\alpha} = 0.05$, $\tilde{R}=10^3 R_{\rm s}$, $m_{\rm BH} = 150 M_\odot$.