First Detection of Secondary Supermassive Black Hole in a Well-Known Binary System

Newswise — Enormous black holes, with a mass exceeding several billion times that of our Sun, exist within the central regions of active galaxies. Scientists study them as luminous galactic hubs where the supermassive black hole consumes material from a turbulent disk known as an accretion disk. A portion of this material is expelled as a formidable jet. This sequence generates a radiant glow from the galactic hub, spanning the full electromagnetic range.

During a recent investigation, scientists discovered indications of a pair of massive black holes orbiting one another by analyzing signals emanating from the jets linked to the accretion process occurring in both black holes. The celestial object under scrutiny, known as OJ287 or a quasar in scientific terms, stands out as a binary black hole system that has been extensively examined and comprehended. When observed in the sky, these black holes are positioned in such close proximity that they blend into a singular point. However, the existence of two distinct black holes becomes evident through the detection of two distinct signal varieties emitted from this dot.

The active galaxy OJ 287 resides within the Cancer constellation and has been under astronomers' scrutiny since 1888, despite being situated approximately 5 billion light-years away. Over 40 years ago, Aimo Sillanpää, an astronomer from the University of Turku, and his colleagues made a notable observation regarding the galaxy's emissions. They discerned a prominent pattern characterized by two distinct cycles: one spanning roughly 12 years, and the other longer, lasting around 55 years. Their proposition was that these cycles originated from the orbital movement of two black holes revolving around each other. The shorter cycle corresponds to the orbital period, while the longer cycle is the consequence of a gradual evolution in the orbit's orientation.

The motion of the black holes in their orbit becomes apparent through a sequence of flares that occur when the secondary black hole periodically crosses the accretion disk of the primary black hole. This crossing transpires at velocities slightly below the speed of light. As the secondary black hole plunges through the disk, it heats the surrounding material, leading to the release of expanding bubbles composed of hot gas. These bubbles, which remain hot for several months, gradually cool down while emitting radiation. This cooling process gives rise to a luminous and intense burst of light known as a flare. Lasting approximately two weeks, these flares outshine a trillion stars in brightness.

Following numerous years of persistent endeavors to determine the precise timing of the secondary black hole's passage through the accretion disk, a team of astronomers led by Mauri Valtonen from the University of Turku, Finland, and his collaborator Achamveedu Gopakumar from the Tata Institute of Fundamental Research in Mumbai, India, along with other researchers, achieved success. They developed a comprehensive model of the orbit, enabling them to accurately predict the occurrence of these flares.

Through a series of triumphant observational campaigns conducted in 1983, 1994, 1995, 2005, 2007, 2015, and 2019, the research team had the opportunity to witness the anticipated flares and validate the existence of a binary supermassive black hole within OJ 287. These observations provided compelling evidence supporting their earlier predictions.

According to Professor Achamveedu Gopakumar, the number of predicted flares has reached a total of 26, and almost all of them have been successfully observed. The larger black hole within this binary system possesses a mass exceeding 18 billion times that of our Sun, while its companion is approximately 100 times lighter. It is worth noting that their orbit is characterized by an elongated shape, deviating from a perfect circle. These findings provide valuable insights into the unique dynamics of the black hole pair within OJ 287.

Despite the diligent efforts made by astronomers, direct signals from the smaller black hole had remained elusive until 2021. Its presence had been inferred primarily through indirect means, such as the observations of flares and the noticeable wobbling effect it imposes on the jet of the larger black hole. This indirect evidence had been instrumental in deducing the existence of the smaller black hole prior to 2021.

The lead author, Professor Mauri Valtonen, highlights the challenging nature of observing the two black holes within OJ 287. Their proximity in the sky causes them to merge into a single point when viewed through telescopes. As a result, it is only when distinct and separate signals from each black hole can be clearly observed that astronomers can confidently claim to have visually witnessed both black holes. This emphasizes the need for precise and distinguishable signals in order to confirm the presence of both black holes within the system.

Smaller black hole directly observed for the first time

Excitingly, the recent observational campaigns conducted on OJ 287 during 2021 and 2022 involved a significant number of telescopes with diverse capabilities. These campaigns yielded groundbreaking results, as researchers were able to capture observations of the secondary black hole making its way through the accretion disk for the first time. Moreover, the signals originating directly from the smaller black hole itself were successfully detected. This represents a significant milestone in the study of OJ 287, providing direct evidence of the presence and behavior of both black holes within the system.

The period spanning 2021 and 2022 held particular importance in the study of OJ 287, as it was anticipated that the secondary black hole would traverse the accretion disk of its larger counterpart during this timeframe. This predicted event was expected to generate an intense blue flash immediately following the impact. Remarkably, the observation of this flash, occurring just days after the projected time, was successfully carried out by Martin Jelinek and his colleagues from the Czech Technical University and Astronomical Institute of Czechia. This precise confirmation of the predicted event provides further validation of the scientific understanding and models surrounding OJ 287.

Intriguingly, the observational campaigns brought forth two significant surprises in the form of previously undetected flare types. The first of these surprises was unveiled through an extensive observation campaign led by Staszek Zola and his team from the Jagiellonian University of Cracow, Poland. The team's meticulous observations led to the discovery of a remarkable event—a massive flare that emitted light equivalent to 100 times the luminosity of an entire galaxy. Astonishingly, this extraordinary flare endured for a mere 24 hours, making it an exceptionally brief yet luminous phenomenon. The discovery of such novel flare characteristics adds a new dimension to our understanding of the behavior and dynamics within OJ 287.

According to the estimates, the occurrence of the flare aligns closely with the moment when the smaller black hole, during its plunge, acquired a substantial amount of new gas to consume. It is this process of gas consumption that triggers the sudden surge in brightness within OJ 287. Additionally, it is believed that this ingestion process enhances the power of the jet emanating from the smaller black hole within OJ 287. Remarkably, although a similar event was predicted a decade ago, its confirmation has only been achieved recently. This discovery sheds new light on the mechanisms at play within the OJ 287 system and deepens our understanding of the complex interactions between black holes and their surrounding environments.

In another surprising revelation, a second unanticipated signal emerged in the form of gamma rays, which were observed by NASA's Fermi telescope. The most significant gamma ray flare recorded in OJ 287 in six years coincided precisely with the occurrence of the smaller black hole's plunge through the gas disk surrounding the primary black hole. This intriguing phenomenon occurs due to the interaction between the jet emitted by the smaller black hole and the gas within the disk, resulting in the generation of gamma rays. To validate this concept, researchers cross-checked their findings with a previous gamma ray flare that transpired in 2013 when the small black hole last passed through the gas disk from the same viewing perspective. The consistent observations further strengthen the understanding of the correlation between the black hole's motion, disk interaction, and the emission of gamma rays in the OJ 287 system.

The reason behind the previously unseen one-day burst lies in unfortunate circumstances rather than a lack of observation. OJ 287 has been documented in photographs dating back to 1888 and has been subject to extensive monitoring since 1970. However, it appears that a stroke of bad luck prevailed. No observers happened to capture OJ 287 on the specific nights when it exhibited its brief one-day stunt. Had it not been for the diligent and dedicated monitoring efforts of Zola's group, this extraordinary event would have gone unnoticed once again. This underscores the significance of continuous and meticulous observation in unraveling the dynamic nature of celestial phenomena like OJ 287.

The extensive efforts directed towards OJ 287 establish it as the leading contender for a supermassive black hole pair capable of emitting gravitational waves in the nano-hertz frequency range. To explore this possibility further, OJ 287 is under regular monitoring by both the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) consortia. These monitoring initiatives aim to uncover additional evidence supporting the existence of a supermassive black hole pair at the core of OJ 287. Particularly, there is a focused endeavor to obtain a radio image of the secondary jet, which would provide invaluable insights into the intricate dynamics of the system. The combined efforts of these consortia promise to enhance our understanding of supermassive black hole pairs and their gravitational wave emissions.

The instruments that were part of the 2021-2022 campaign include NASA’s Fermi gamma ray telescope and the Swift ultraviolet to x-ray telescope, optical wavelength observations by astronomers in Czech Republic, Finland, Germany, Spain, Italy, Japan, India, China, Great Britain and USA, and radio frequency observations of OJ287 at Aalto University, Helsinki, Finland.