Electromagnetic Flares from Compact-Object Mergers in AGN Disks: Signatures and Predictions

TL;DR

This paper explores electromagnetic signatures from black hole mergers in AGN disks, predicting observable gamma-ray, X-ray, and optical flares that could accompany gravitational wave detections.

Contribution

It introduces a model for post-merger EM flares in AGN disks, linking jet breakout and shock emissions to observable signals, and addresses BH growth concerns.

Findings

Jet breakout produces detectable gamma-ray emission.

UV and optical emissions last from an hour to a month.

Model reproduces gamma-ray and optical flares similar to GW-associated signals.

Abstract

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 post-merger 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 $\sim10^3$ AGNs with luminosities ranging from $\sim 10^{44}$ to $\sim 10^{45}~{\rm erg~s^{-1}}$. With a single set of parameters, this post-merger 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.