The blazar OJ 287 may have provided key information about active galactic nuclei

Artist's concept of the blazar OJ 287
Artist’s concept of the blazar OJ 287

An article published in the journal “Monthly Notices of the Royal Astronomical Society” describes a research on a blazar, a type of active galactic nucleus, known as OJ 287. A team of researchers led by Silke Britzen of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, studied this blazar, which has long been known and left the astronomers puzzled by its variations in brightness. The cause could be in the presence of two black holes or a misaligned accretion disk.

Approximately 3.5 billion light years away from the Earth, the blazar OJ 287 is powered by a supermassive black hole that is active due to a large amount of materials orbiting it that get heated enough to emit intense amounts of electromagnetic radiation. When the materials get swallowed, a part gets ejected in jets at the poles that reach very high speeds. The emissions of OJ 287 are variable and were identified for the first time in 1891 then they discovered that this blazar had already appeared in photographic plates taken in 1887.

Over the subsequent decades, many more observations of the blazar OJ 287 were carried out with increasingly sophisticated instruments that allowed to detect the jets, called in jargon knotes, coming from the supermassive black hole. These are plasma flows whose emissions reach the Earth in the form of radio waves and vary in brightness due to a mechanism that was thought to be related to the black hole’s feeding. The jets movements have characteristics that were attributed to shock waves traveling inside them. The problem was to understand the connection between the two phenomena.

Silke Britzen’s team used various radio telescopes coordinated to form a single much larger virtual instrument thanks to the technique called interferometry which allows for much greater detail than a single radio telescope. A total of 120 observations conducted between April 1995 and April 2017 provided an explanation for the changes in brightness and movements of the plasma jets in a precession movement.

The plasma jets move at such speed that the relativistic effects are significant and then what in jargon is called “Doppler boosting” can be applied, an effect that modifies the apparent luminosity of the source, in this case of the jets. This effect is the result of the precession, which determines changes in the angle of observation. At the same time, the jet precedes and seems to also follow another smaller movement similar to the nutation. Their combination determines the variability of the radio emissions.

The precession is a phenomenon that also affects the Earth, whose axis of rotation moves over time in a cycle of 26,000 years in a movement similar to that of a spinning top. In the case of OJ 287, the researchers indicated two possible processes in progress, illustrated in the image (courtesy Axel M. Quetz/MPIA Heidelberg. All rights reserved): in the inset A there are two supermassive black holes that generate tidal effects with the precession as a consequence, in the inset B the accretion disk that orbits a single supermassive black hole is misaligned and in this case it’s their interaction that generates the precession.

A binary supermassive black hole is very rare so if that’s the correct hypothesis it would be an even more interesting discovery. It’s not the first time that is proposed, on the contrary another team of researchers explained it to the 211st meeting of the American Astronomical Society (AAS) in 2008. According to that team, there’s a main supermassive black hole with an mass estimated at 18 billion times the Sun’s and a secondary one with a mass estimated at 100 million times the Sun’s.

Regardless of the precession’s cause, according to Silke Britzen’s team that process can also explain the visible light emissions detected in 130 years but the verification of this hypothesis will require further study. However, the researchers believe they achieved an excellent result, so much so that in the article they talk about a Rosetta stone for blazars. In fact, they believe that OJ 287 allowed them to decipher the mechanism of blazars just like the Rosetta stone allowed to decipher the Egyptian hieroglyphs.

The activity of supermassive black holes has a strong influence on their host galaxies, beginning with that on star formation, so their study is important. If they could prove that it’s a binary black hole it would be even more interesting thinking of the rising branch of gravitational wave-based astronomy. A next-generation gravitational wave detector could be sensitive enough to detect the ones emitted by a pair of supermassive black holes even before they merged.

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