Observations by the East Asian VLBI Network have revealed periodic motions in the jet emanating from the center of the elliptical galaxy M87−specifically, a precessional motion with a cycle of approximately 11 years and a lateral wobble with a cycle of about 0.9 years. An international research team from Kogakuin University, the National Astronomical Observatory of Japan (NAOJ), Bunkyo University, the Korea Astronomy and Space Science Institute (KASI), Central China Normal University, Nagoya City University, and Kyung Hee University hypothesized that these periodicities could be caused by the orbital motion of a second black hole revolving around the supermassive black hole at the center of M87. They theoretically derived the possible mass range of this hypothetical secondary black hole. Recently, the NANOGrav collaboration suggested that gravitational waves emitted by supermassive black hole binaries may have been detected as a “gravitational wave background” permeating the universe. Motivated by this, the research team focused on M87 as a promising candidate to explore this phenomenon. Their findings offer important guidance for future observational strategies aimed at directly testing the existence of supermassive black hole binaries.

In June 2023, the NANOGrav collaboration in the United States analyzed 15 years of pulsar timing array data and reported evidence for the presence of low−frequency gravitational waves oscillating over long timescales (Note 1). These gravitational waves are believed to constitute a “gravitational wave background” arriving from all directions in the universe. One of the most likely sources of such waves is believed to be supermassive black hole binaries formed as a result of galaxy mergers.

When two galaxies merge, their central supermassive black holes gradually approach each other and eventually form a binary system that orbits around a common center of mass. The gravitational waves emitted during this process are thought to make up a major component of the gravitational wave background. According to theoretical models of galactic mergers and evolution, supermassive black hole binaries closer to Earth are expected to contribute more significantly to this background.

Building on this understanding, the research team turned their attention to the elliptical galaxy M87, located about 55 million light−years from Earth. M87 gained widespread recognition when the Event Horizon Telescope (EHT) captured the first−ever image of a black hole's shadow. In recent years, long−term observations using the East Asian VLBI Network (EAVN, Figure 1) have led to the discovery of an 11−year periodic precession in the direction of M87's radio jet (Figure 2).
Additionally, data from EAVN have revealed a lateral oscillatory motion with a roughly 0.9−year cycle.

In this study, the team theorized what the mass range of a hypothetical secondary black hole would be if these periodic motions were caused by orbital dynamics within a supermassive black hole binary (Figure 3). Their analysis imposed conditions such as: the gravitational waves generated by this hypothetical binary in M87 must not exceed the intensity of the gravitational wave background observed by NANOGrav, the two black holes have not yet merged, and the orbital radius of the primary black hole must be larger than its gravitational radius. Based on these constraints, they determined that for an 11−year orbital period, the secondary black hole's mass must range from 0.7% to 4.2% of the primary's mass (approximately 6.5 billion solar masses), and for a 0.9−year period, the range would be 3.7% to 100% (Figure 4). These results offer crucial direction for future observational strategies aimed at directly detecting supermassive black hole binaries.

Motoki Kino of Kogakuin University, the lead author of the study, stated:
“Theoretical models of galaxy mergers and evolution suggest that nearby supermassive black hole binaries contribute significantly to the gravitational wave background. This was the motivation for our research. By precisely measuring the root of M87's jet with high resolution in future observations, we might be able to directly confirm the existence of a black hole binary.”

Masahiro Nagashima of Bunkyo University commented:
“The history of galactic mergers and evolution is closely linked to the growth of black holes. Thus, closely examining M87−a nearby supermassive black hole−is a fundamental and vital step toward understanding galaxy evolution across the universe.”

Kazuhiro Hada of Nagoya City University added:
“The East Asian VLBI Network is now actively developing the 86 GHz band. This band will allow us to see even deeper into the base of the jet with higher resolution and transmission rate. It will play an essential role in verifying the existence of a supermassive black hole binary system.”

The research findings were published in The Astrophysical Journal on June 5, 2025, under the title: “Constraining the Mass of a Hypothetical Secondary Black Hole in M87 with the NANOGrav 15 yr Data Set” by Kino et al.

Figure

Figure 1: East Asian VLBI Network
The East Asian VLBI Network is an international very long baseline interferometry (VLBI) array jointly developed by Japan, South Korea, China, and others. Using a system of radio telescopes spaced over distances greater than 5,000 km, it enables high−resolution imaging of radio emissions from massive black holes. (Click to enlarge)

Figure 2: M87 Jet Observed by EAVN
(Top) Example of a radio image of the M87 jet captured by EAVN. Arrows indicate the jet&aposls direction of ejection. (Bottom) Time series of jet ejection directions measured from 2000 to 2022. The red curve shows a sine wave with an 11−year period, which closely matches the observed directional variation. (Click to enlarge)

Figure 3: Conceptual Diagram of Orbital Motion with a Hypothetical Second Black Hole
The black hole at the base of the M87 jet observed by VLBI is assumed to be the primary black hole. Its orbital motion around a shared center of mass with a secondary black hole is thought to cause the jet's periodic oscillations. (Click to enlarge)

Figure 4: Permissible Mass Range for a Hypothetical Secondary Black Hole in M87
The blue segments indicate allowed mass ratios of the secondary to primary black hole. The gray−shaded area shows excluded regions that do not meet the binary system criteria. (Click to enlarge)

Note 1:

NANOGrav (North American Nanohertz Observatory for Gravitational Waves) is a North American research project aimed at detecting nanohertz−frequency gravitational waves using pulsar timing arrays. By precisely measuring the arrival times of radio signals from multiple millisecond pulsars distributed across the galaxy, the technique−essentially a “galaxy−scale gravitational wave telescope”−detects minuscule fluctuations in spacetime.

Journal Reference

• Title:
  Constraining the Mass of a Hypothetical Secondary Black Hole in M87 with the NANOGrav 15 yr Data Set
• Authors:
  Motoki Kino, Masahiro Nagashima, Hyunwook Ro, Yuzhu Cui, Kazuhiro Hada, and Jongho Park
• Published in:
  The Astrophysical Journal (2025) 986, 49
• https://iopscience.iop.org/article/10.3847/1538-4357/adceb6