Effelsberg radio telescope sheds new light on binary black holes

BY DR NORBERT JUNKES (MAX PLANCK INSTITUTE FOR RADIO ASTRONOMY)

Multi-wavelength observations involving Germany’s Effelsberg 100 m radio telescope have provided important new insights into the binary black hole at the centre of the active galaxy OJ 287.

Using the densest and longest radio-to-high-energy observations to date, scientists tested crucial binary model predictions and, for the first time, an independent black hole mass determination of the system was performed. The results have been published in the Monthly Notices of the Royal Astronomical Society: Letters and The Astrophysical Journal

When two galaxies collide and merge, supermassive binary black holes are formed. These binaries a play a key role in the evolution of galaxies and the growth of supermassive black holes. Furthermore, coalescing binaries are the Universe’s loudest sources of gravitational waves.

The blazar OJ 287 is one of the best candidates for hosting a compact binary supermassive black hole. A blazar is observed when a black hole consuming large amounts of gas and dust at the centre of a galaxy emits jets of plasma travelling at close to the speed of light which are pointing towards Earth.

Exceptional outbursts of radiation which repeat every 11 to 12 years are OJ 287’s claim to fame. Some of these are so bright that OJ 287 temporarily becomes the brightest source of its type in the sky. As the second black hole in the system orbits the other more massive one, its motion affects the radiation that is observed from its partner’s jet and/or accretion disk, dimming the light output in a semi-periodic way.

However, until now there has been no direct independent determination of the mass of the more massive black hole in the system. None of the models could be critically tested in systematic observing campaigns because these campaigns lacked broadband coverage involving radiation of many different frequencies. Now, multiple simultaneous X-ray, UV and radio observations, along with optical and gamma-ray bands, have been used.

“OJ 287 is an excellent laboratory for studying physical processes in an extreme environment: disks and jets of matter in the immediate vicinity of one or two supermassive black holes,” said Dr Stefanie Komossa of the Max Planck Institute for Radio Astronomy, who led the research group. “Therefore, we started high-cadence observations of OJ 287 at more than 14 frequencies from the radio to the high energy regime lasting for years.”

BELOW: A deep ultraviolet image of OJ 287 and its environment (left), and artist’s impression of the very centre, including the accretion disk, the jet, and a second black hole orbiting the primary black hole (right).

Credit: S. Komossa et al.; NASA/JPL-Caltech

The mass of the primary black hole was derived from the motion of gaseous matter bound to the black hole, amounting to 100 million times the mass of our Sun.

“This result is very important, as the mass is a key parameter in the models studying the evolution of this binary system: how far are the black holes separated, how quickly will they merge, how strong is their gravitational wave signal?” said Dr Dirk Grupe, associate professor at Northern Kentucky University and a coauthor on both papers.

The new results imply that an exceptionally large mass of the black hole of OJ 287, exceeding 10 billion solar masses, is no longer required. The results rather favour a binary model of more modest mass.

The study resolves two old puzzles: the apparent absence of the latest of the bright outbursts which OJ 287 is famous for, and the emission mechanism behind the outbursts. The observations allow for the precise timing of the latest outburst; it did not occur in October 2022, as predicted by the “hugemass” model, but rather in 2016-2017. Radio observations with the Effelsberg 100 m telescope reveal that these outbursts are non-thermal in nature, implying that jet processes are the power source of the outbursts.

The results affect ongoing and future search strategies for binary systems using major facilities such as the Event Horizon Telescope and, in the future, the SKA telescopes. They could enable direct radio detection and spatial resolution of the binary sources in OJ 287 and similar systems, as well as the detection of gravitational waves from these systems.

Read the papers: https://iopscience.iop.org/article/10.3847/1538-4357/acaf71

https://academic.oup.com/mnrasl/article/522/1/L84/7044769?login=false