A Light in the Dark: Searching for Electromagnetic Counterparts to Black Hole-Black Hole Mergers in LIGO/Virgo O3 with the Zwicky Transient Facility

Matthew J. Graham, Barry McKernan, K. E.Saavik Ford, Daniel Stern, S. G. Djorgovski, Michael Coughlin, Kevin B. Burdge, Eric C. Bellm, George Helou, Ashish A. Mahabal, Frank J. Masci, Josiah Purdum, Philippe Rosnet, Ben Rusholme

Research output: Contribution to journalArticlepeer-review


The accretion disks of active galactic nuclei (AGNs) are promising locations for the merger of compact objects detected by gravitational wave (GW) observatories. Embedded within a baryon-rich, high-density environment, mergers within AGNs are the only GW channel where an electromagnetic (EM) counterpart must occur (whether detectable or not). Considering AGNs with unusual flaring activity observed by the Zwicky Transient Facility (ZTF), we describe a search for candidate EM counterparts to binary black hole (BBH) mergers detected by LIGO/Virgo in O3. After removing probable false positives, we find nine candidate counterparts to BBH mergers during O3 (seven in O3a, two in O3b) with a p-value of 0.0019. Based on ZTF sky coverage, AGN geometry, and merger geometry, we expect ≈3(N BBH/83)(f AGN/0.5) potentially detectable EM counterparts from O3, where N BBH is the total number of observed BBH mergers and f AGN is the fraction originating in AGNs. Further modeling of breakout and flaring phenomena in AGN disks is required to reduce our false-positive rate. Two of the events are also associated with mergers with total masses >100 M , which is the expected rate for O3 if hierarchical (large-mass) mergers occur in the AGN channel. Candidate EM counterparts in future GW observing runs can be better constrained by coverage of the Southern sky as well as spectral monitoring of unusual AGN flaring events in LIGO/Virgo alert volumes. A future set of reliable AGN EM counterparts to BBH mergers will yield an independent means of measuring cosmic expansion (H 0) as a function of redshift.

Original languageEnglish (US)
Article number99
JournalAstrophysical Journal
Issue number2
StatePublished - Jan 1 2023

Bibliographical note

Funding Information:
The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant No. 12540303 (PI: Graham).

Funding Information:
Based on observations obtained with the Samuel Oschin Telescope 48 inch and the 60 inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility (ZTF) project. ZTF is supported by the National Science Foundation under grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW.

Funding Information:
This work was supported in part by the National Science Foundation grant Nos. AST-1815034, AST-1831412, and AST-2108402, the NASA grant No. 16-ADAP16-0232, and Simons Foundation grant No. 533845. The work of D.S. was carried out at Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. M.W.C. is supported by the National Science Foundation with grant Nos. PHY-2010970 and OAC-2117997. P.R. acknowledges the support received from the Agence Nationale de la Recherche of the French government through the program “Investissements d’Avenir” (16-IDEX-0001 CAP 20-25).

Funding Information:
The authors thank Will Farr and Colm Talbot for very useful discussions about LIGO/Virgo waveform choices and parameterization. M.J.G., B.M., and K.E.S.F. acknowledge the Center for Computational Astrophysics at the Flatiron Institute, New York for their hospitality and support.

Publisher Copyright:
© 2023. The Author(s). Published by the American Astronomical Society.


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