[Paper Review] AGN (and other) astrophysics with Gravitational Wave Events

K. E. Saavik Ford, I. Bartos+15 more|arXiv (Cornell University)|Mar 22, 2019

Pulsars and Gravitational Waves Research6 references4 citations

TL;DR

This paper proposes that gravitational wave (GW) detections by LIGO can statistically identify stellar-mass black hole binary (sBBH) mergers originating in active galactic nucleus (AGN) disks, enabling measurement of key disk properties like density, aspect ratio, and lifetime. By cross-correlating LIGO event localizations with multi-wavelength galaxy catalogs, the study shows that ~100 GW detections could confirm an AGN origin with 95% confidence, even if only a fraction of mergers occur in AGNs.

ABSTRACT

The stellar mass binary black hole (sBBH) mergers presently detected by LIGO may originate wholly or in part from binary black hole mergers embedded in disks of gas around supermassive black holes. Determining the contribution of these active galactic nucleus (AGN) disks to the sBBH merger rate enables us to uniquely measure important parameters of AGN disks, including their typical density, aspect ratio, and lifetime, thereby putting unique limits on an important element of galaxy formation. For the first time, gravitational waves will allow us to reveal the properties of the hidden interior of AGN disks, while electromagnetic radiation (EM) probes the disk photosphere. The localization of sBBH merger events from LIGO is generally insufficient for association with a single EM counterpart. However, the contribution to the LIGO event rate from rare source types (such as AGNs) can be determined on a statistical basis. To determine the contribution to the sBBH rate from AGNs in the next decade requires: {\em 1) a complete galaxy catalog for the LIGO search volume, 2) strategic multi-wavelength EM follow-up of LIGO events and 3) significant advances in theoretical understanding of AGN disks and the behavior of objects embedded within them.}

Motivation & Objective

• To determine whether stellar-mass black hole binary (sBBH) mergers detected by LIGO originate in active galactic nucleus (AGN) disks.
• To use statistical cross-correlation between LIGO GW event localizations and galaxy catalogs to constrain the contribution of AGNs to the sBBH merger rate.
• To enable unique constraints on AGN disk parameters—such as density, aspect ratio, and lifetime—using gravitational wave data.
• To guide multi-wavelength EM follow-up campaigns for LIGO events to identify rare, AGN-associated counterparts.
• To advocate for coordinated advances in EM surveys, theoretical modeling of embedded objects, and next-generation detectors to maximize multi-messenger insights.

Proposed method

• Use statistical cross-correlation between LIGO GW event error volumes and galaxy catalogs to detect excess spatial association with AGNs.
• Apply a 3σ significance threshold and 95% confidence interval to detect a non-zero contribution of AGNs to the sBBH merger rate.
• Model the expected number of GW detections required to identify AGN-originated mergers based on AGN number density (n_AGN ~ 10⁻⁴–10⁻⁵ Mpc⁻³) and merger fraction (d_AGN).
• Integrate multi-wavelength EM data (radio, optical, UV, X-ray) to identify EM counterparts and distinguish AGN activity from star formation.
• Leverage future surveys (e.g., VLASS, SKA, NGVLA, Athena) and high-time-resolution UV/X-ray observations to probe disk physics and detect low-mass companions.
• Combine GW data with high-energy neutrino (HEN) searches (e.g., IceCube-Gen2) to improve localization and identify common sources.

Experimental results

Research questions

• RQ1 What fraction of LIGO-observed sBBH mergers originate in AGN disks, and can this be measured statistically?
• RQ2 Can the presence of AGN disks be uniquely constrained by cross-correlating GW events with galaxy catalogs, despite poor localization?
• RQ3 What EM signatures (e.g., radio, UV, X-ray) can be used to distinguish AGN disk activity from other galaxy types in LIGO error volumes?
• RQ4 How can future EM surveys and detectors improve the identification of AGN-associated GW events?
• RQ5 What constraints can be placed on AGN disk parameters (density, aspect ratio, lifetime) using GW data alone?

Key findings

• As few as ~100 LIGO GW detections could statistically confirm a non-zero contribution of AGNs to the sBBH merger rate at 95% confidence, assuming d_AGN = 1 (all mergers in AGNs).
• With AGN number density n_AGN ~ 10⁻⁴–10⁻⁵ Mpc⁻³, a 3σ excess in spatial correlation with LIGO error volumes can be detected within a few years of LIGO A+ operation.
• UV follow-up with space-based FUV/NUV spectroscopy (R > 100, effective area ≥ 1 m²) could detect short-timescale variability in hundreds of AGNs (z < 0.6) in 10 hours at 5σ significance.
• X-ray observations with energy resolution ~2–6 eV in the Fe Kα band can detect radial velocity shifts from low-mass companions (q ~ 0.01) around supermassive black holes.
• An effective area of ≥2.5 m² at 6 keV for Fe Kα line studies is needed to build a statistically significant sample of broad lines around SMBHs.
• Combining GW data with high-energy neutrino searches (e.g., IceCube-Gen2) improves source localization and increases discovery potential for multi-messenger sources.