Light flare brighter than a trillion suns points to a binary black hole system

When observed through a telescope, the two black holes at the center of galaxy OJ 287 are so close together that they blend into a single dot. The researchers, however, noticed two distinct signals, brighter than a trillion suns, coming from the galactic center, which led them to this discovery.

The history of Galaxy OJ 287

Astronomers have been observing OJ 287 since its discovery in 1888. The galaxy exhibits a prominent pattern characterized by two distinct cycles: one lasting approximately 12 years and the other lasting 55 years. These cycles were initially interpreted as the orbital movement of two black holes revolving around each other.

The motion of the black holes in their orbit becomes apparent through a sequence of flares that occur when the second black hole periodically crosses the accretion disk of the primary black hole. These flares, lasting approximately two weeks, outshine a trillion stars in brightness. 

However, scientists have now observed both black holes directly.

How flares helped them to detect the smaller black hole

The binary black hole system has a massive black hole with a mass greater than 18 billion times that of the Sun and a smaller black hole around 100 times lighter.

Scientists detected the bigger black hole from the observation of flares, but the detection of the smaller one required more nuance. 

Observational campaigns conducted in 2021 and 2022, led by Staszek Zola from the Jagiellonian University of Cracow in Poland, helped the team to observe the smaller black hole directly.

The key was to have the precise timing of its passage through the accretion disk of the primary black hole. This was achieved thanks to a comprehensive model of the orbit developed by the team. This allowed them to accurately predict the occurrence of flares associated with crossing the accretion disk.

In 2021, an intense blue flash confirmed the presence of the second black hole in OJ 287. Additionally, the observational campaigns revealed a brief but bright flare, emitting light equivalent to 100 times the luminosity of a galaxy, discovered by the team led by Zola.

Around the same time, NASA's Fermi telescope detected gamma rays from the smaller black hole due to its plunge through the accretion disk. 

The combined observations led scientists to confirm the presence of the smaller black hole in the binary system. This milestone provided direct evidence of both black holes and their behavior within the system.

The research findings were published in Monthly Notices of the Royal Astronomical Society. 

Study abstract:

The bright blazar OJ 287 routinely parades high brightness bremsstrahlung flares, which are explained as being a result of a secondary supermassive black hole (SMBH) impacting the accretion disc of a more massive primary SMBH in a binary system. The accretion disc is not rigid but rather bends in a calculable way due to the tidal influence of the secondary. Next, we refer to this phenomenon as a variable disc level. We begin by showing that these flares occur at times predicted by a simple analytical formula, based on general relativity inspired modified Kepler equation, which explains impact flares since 1888. The 2022 impact flare, namely flare number 26, is rather peculiar as it breaks the typical pattern of two impact flares per 12-yr cycle. This is the third bremsstrahlung flare of the current cycle that follows the already observed 2015 and 2019 impact flares from OJ 287. It turns out that the arrival epoch of flare number 26 is sensitive to the level of primary SMBH’s accretion disc relative to its mean level in our model. We incorporate these tidally induced changes in the level of the accretion disc to infer that the thermal flare should have occurred during 2022 July–August, when it was not possible to observe it from the Earth. Thereafter, we explore possible observational evidence for certain pre-flare activity by employing spectral and polarimetric data from our campaigns in 2004/05 and 2021/22. We point out theoretical and observational implications of two observed mini-flares during 2022 January–February.