For the first time scientists have been able to ‘see’ the secondary black hole in the binary system of a unique blazar 5.2 billion light years away by measuring polarized light from it.

Analyses of optical observations of the blazar called OJ 287 since 1980 had earlier shown that it exhibited periodic outburst around every 12 years and the finding was explained by proposing OJ 287 as a binary black hole system. The next outburst was predicted in 1994 - 1995 which was detected and later improved models show these were double peaked outbursts.  More double peaked outbursts were predicted from 2015 – 2021.

In order to capture the stellar fireworks, Dr Alok Gupta, Scientist at Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institute of Department of Science and Technology, initiated an international monitoring observation campaign in optical bands. An international group of 28 scientists from (China, India, Finland, Russia, USA, Japan, and Bulgaria) carried out the most extensive and densely sampled optical flux and polarization observations in the history of 150 years of this source. They found evidence of relativistic charged particles jet emission from the secondary black hole (less massive, million solar mass black hole) which is only away by about 10 micro arcseconds from the primary black hole (more massive, billion solar mass black hole).

OJ 287 is recognized as a binary black hole system where the orbit slowly spirals in because of energy lost to gravitational radiation. Previously the signals arising from the binary system have been associated either with the primary black hole, an ultramassive black hole, or with the accretion disk of gas surrounding it. The recent study published in The Astrophysical Journal Letters has added new evidence to this scenario by observing signals that arise from the smaller (secondary) black hole directly.

The study makes the researchers confident that OJ 287 is really an ultramassive binary black hole system, where signals from both components can be separated, in spite of their closeness to each other in the sky. It has collected enough observational signatures that can give evidence of the black holes in the binary system and their properties and  could establish the blazar system as one of the first potential gravitational wave binary supermassive black hole system for possible detection of Gravitational Waves (GWs) using space based GW experiments and Pulsar Timing Array (PTA).

Publication link: https://doi.org/10.3847/2041-8213/acfd2e

For more details, please contact Dr. Alok C. Gupta on alok[at]aries[dot]res[dot]in or acgupta30[at]gmail[dot]com.

Figure 1: The historical light curve of OJ 287 in the optical V-band (on the left) and the binary BH picture of its central engine (on the right). The black holes are detected by the light emitted by their jets, both of which point to our direction. The drawing in the right panel of the figure is adapted from a drawing by Pauli Pihajoki. A closely similar drawing has appeared in Valtonen et al. 2023, MNRAS, 525, 1153.