An international team of researchers featuring scientists from the JU Astronomical Observatory has confirmed the presence of two supermassive black holes in the centre of the OJ 287 galaxy. Their existence was first suggested in 1982 by astronomers from the University of Turku in Finland. Now the first clear observations of the smaller of the two black holes have been made.

The main black hole, OJ 287, which has a mass of about 18 billion Suns, is surrounded by a galactic accretion disk from which it is slowly ‘sucking in’ matter. This supermassive black hole also generates jets, i.e. high-energy, highly collimated outflows of matter accompanied by electromagnetic radiation that can be observed by telescopes on Earth. A second, smaller black hole with a mass of ‘only’ 150 million solar masses orbits its companion and periodically breaks through its disk, causing a sudden, brief flash of brightness throughout the galaxy. The temporal variability of the brightness of the entire object has been observed for 120 years, and its earliest photos were recorded on glass photographic plates.

TESS orbital telescope mission

NASA's Transiting Exoplanet Survey Satellite (TESS) orbital telescope searches for exoplanets on a daily basis. So far, it has detected 410 of them, while the total number of known globes (other than the Sun) orbiting stars in our Galaxy is estimated at over 5,500. However, in 2021, TESS spent several weeks closely observing an object of a completely different nature: it was pointed precisely at the OJ 287 galaxy to help confirm the hypothesis that there are two black holes at its centre. To detect signs of the less massive one, the telescope monitored the brightness of the main black hole and its associated jet.

Flare detected by TESS

We currently have no way of directly observing the smaller black hole orbiting the larger one, but its presence was revealed by the galaxy's sudden increase in brightness. The brightening itself lasted only 12 hours, showing how difficult it is to record such an event unless the time it happens is known in advance. Moreover, in the case of OJ2 87, this particular type of brightening has never been observed before. Still, it was expected, since Pauli Pihajoki from the University of Turku predicted it in his doctoral thesis back in 2014. According to him, the flare was to occur in late 2021, giving scientists around the world enough time to plan observations of the galaxy using multiple ground-based and space-based telescopes. The TESS satellite detected the expected flare on 12 November 2021 at 2.00 GMT. The discovery was independently confirmed by others, including the Swift Multi-Range Orbital Observatory. This way, scientists found indirect but clear evidence that the less massive black hole OJ 287 orbits a giant black hole about 100 times larger than itself.

JU Astronomical Observatory flare observations

An international research group led by Prof. Stanisław Zoła from the JU Astronomical Observatory detected the same event using Skynet telescopes located in different parts of the world, maintaining the continuity of observations regardless of the time of day and night. This network also includes one of the large optical telescopes located at the JU Observatory in Kraków. The observations were additionally facilitated by a clear sky. In a newly published paper which brings together all observations to date, Mauri Valtonen and his team in Turku prove that the 12-hour increase in brightness of the OJ 287 galaxy actually came from the smaller black hole and its immediate surroundings. The results of the study were published in The Astrophysical Journal Letters.

Evidence for a less massive black hole

The rapid brightness spike occurs when the smaller black hole absorbs a large portion of the accretion disk surrounding the larger black hole, transforming it and redirecting it into a stream of gas (jet) that flows far outward. Under normal conditions, the yellow jet of the smaller black hole is brighter than the dominant red jet of the larger one for this short period of time. This also makes the colour of OJ 287 less red and more yellow. After several hours, the red colour returns. The results also confirm temporary changes in other features of the light emitted by OJ 287 at the same time.

This is a very convincing piece of evidence for the presence of a less massive black hole. It is worth adding that, as in the case of exoplanets detected by TESS and other telescopes, we currently do not yet have the means that allow for direct imaging. This is due to the enormous distance between us and OJ 287, spanning over four billion light years.

What's next for OJ 287?

Additionally, OJ 287 is all the more interesting for another reason. The research team predicts that two such huge objects orbiting closely together should produce gravitational waves in the nanohertz range. These waves should cause minimal changes in the periods of nearby pulsars, which we may soon be able to measure accurately enough on Earth. The orbital period of the system of two black holes should also shorten over time due to the emission of gravitational waves, and eventually both objects will merge into one.

The described results are part of research conducted at the Department of Stellar and Extragalactic Astronomy of the JU Astronomical Observatory. The article was based on a text authored by Dr Elżbieta Kuligowska (JU Astronomical Observatory).

Original publications:

Mauri J. Valtonen, Staszek Zola, Alok C. Gupta et al., Evidence of Jet Activity from the Secondary Black Hole in the OJ 287 Binary System, The Astrophysical Journal Letters (2024). DOI: 10.3847/2041-8213/ad4d9b;

Shubham Kishore et al, Rapid Optical Flares in the Blazar OJ 287 on Intraday Timescales with TESS, The Astrophysical Journal (2023). DOI: 10.3847/1538-4357/ad0b80.