An article published in “The Astrophysical Journal Letters” reports observations of a supermassive black hole passing through the disk of materials surrounding another object of the same type but even more massive in the galaxy OJ 287. A team of researchers used NASA’s Spitzer space telescope to monitor this event, which was predicted by a model created specifically to take into account the extreme environment generated in particular by the larger of the two black holes, whose mass is estimated at around 18 billion times the Sun’s. This model, from 2018, is the most recent and takes into account gravitational waves but also the no-hair theorem.
About 3.5 billion light-years away from Earth, the OJ 287 galaxy contains a blazar, a type of quasar with a jet of highly-energetic particles aimed at Earth. The blazar is powered by a supermassive black hole surrounded by a disk of gas and dust that get heated to the point of emitting very strong electromagnetic radiation that makes it very bright even at that distance. It’s one of the largest known black holes with an estimated mass of about 18 billion times the Sun’s. OJ 287 has the particularity of having a second supermassive black hole, another huge object that, however, seems small compared to the primary with a mass that is estimated to be “only” 150 million times the Sun’s.
Probably the OJ 287 galaxy is the result of a galaxy merger, and each of the two original galaxies had a supermassive black hole at its center. Now these two black holes got close, and in the future they too will merge, probably within 10,000 years. For now, the black hole that is now secondary is orbiting the primary in an irregular and oblong orbit, and this means that it passes through the disk of materials at different times over 12 years. When this occurs, the impact with the materials generates two expanding hot gas bubbles that move away in opposite directions quadrupling the quasar’s brightness in less than 48 hours.
The outburst generated by the interaction between the secondary supermassive black hole and the disk surrounding the primary has long been known because it can be seen in photos taken at the end of the 19th century. At the time, astronomers had no idea what it was, and only in this century was it possible to create models that predict those events. In 2010 a computer model made it possible to predict an event of that type with an accuracy of between one and three weeks. In 2018 a new model included gravitational waves and the no-hair theorem allowing to increase the precision to the point of predicting the July 31, 2019, event with a 4-hour margin.
The Spitzer space telescope allowed to monitor the OJ 287 blazar at a time when the Sun was between it and the Earth, covering its view to ground-based and Earth-orbiting telescopes. Spitzer worked in an orbit around the Sun until its mission ended on January 30, 2020, and was in a position to observe the outburst in OJ 287 from its beginning, on July 31, until early September, when it faded away and the blazar became visible again from Earth and its orbit.
The birth of gravitational wave astronomy offered a significant contribution to this research because it provided very useful information for the creation of the new model. The results confirm the no-hair theorem which states that black holes have a smooth surface. In essence, the pair of supermassive black holes in the OJ 287 galaxy make a sort of immense space laboratory that allows to test the theory of relativity and other physics models concerning black holes.
