Blazar 3.5 billion light-years from Earth provides better understanding of physics powering their massive optical flare

Scientists tracking a blazar 3.5 billion light-years from Earth that stands out with its quasi-periodic optical outbursts over trillion times the Sun's luminosity, going back approximately 120 years, have traced the reason behind the sudden increase in its flux states. While the source of its optical flare, which was earlier understood to be a binary supermassive black hole, the new study found that the source to be more complex. This study will provide a better understanding of blazars and the physics powering the source of their optical flare.

Blazars are one of the brightest sources in the Universe. A special class of these objects are called BL Lacs, which show rapid and large variability in emission. A blazar called OJ 287, whose central supermassive black hole is among the largest known, belongs to this class. However, the origin of its optical flare is unique and different from other BL Lacs. It had been proposed as a binary black hole system, where one supermassive black hole has been orbiting around the central black hole with an orbital period of almost 12 years (result from century-long optical monitoring). The underlying physical mechanism of optical flaring has remained a puzzle for a long time, mainly because of its unpredictability and huge luminosity.

Studies done in the past on OJ 287 preferred the binary black hole model for this source. But a flare was observed in April-May 2020, which was not predicted under the binary black hole scenario suggesting that there are other physical phenomena involved in this source that are causing the bright X-ray and optical flares which needed to be explored.

A group of scientists from Raman Research Institute, an autonomous institute of the Department of Science & Technology, Govt. of India, along with Rukayia Khatoon from Tezpur University, Prof. Bożena Czerny from Center for Theoretical Physics, Poland and Dr. Pratik Majumdar from Saha Institute of Nuclear Institute studied the blazar OJ 287. In particular they studied the 2nd brightest flare observed in X-ray in April-May 2020 observed very interesting behaviour of the X-ray spectrum during its flaring and non-flaring states. The team consisting of Raj Prince, Gayathri Raman, and Varun, past Ph.D. students of Raman Research Institute, Aditi Agarwal, currently a Postdoctoral fellow at Raman Research Institute, and Nayantara Gupta, a faculty member at Raman Research Institute, also detected a significant spectral change in X-ray and optical-UV suggesting a complex nature of the source of blazar OJ 287.

They included the observational data recorded by Astrosat, the first dedicated Indian astronomy mission aimed at studying celestial sources in X-ray, optical, and UV spectral bands simultaneously, along with publicly available data from other detectors from all over the world like the Swift-XRT/UVOT, NuSTAR to explore the temporal as well as spectral behavior of this source.

They found a significant change in the optical-UV and X-ray spectrum, which leads to a shift in the location of the peak of radiation from highly energetic electrons in the magnetic field or the peak of synchrotron emission towards higher energy. As a result, the blazar OJ 287, which is known to be a BL Lac type object with peak energy flux at low energy, showed a peak at higher energy.

The temporal and the spectral properties of blazar OJ 287 study published in “Monthly Notices of the Royal Astronomical Society (MNRAS)” suggested a significant change in the spectral property as the source travels from a low flux state to a high flux state. The modeling of the observational data suggests an increase in the jet magnetic field (magnetic field in the jet-like emission region) during the flaring state.

Binary black hole systems in blazars are very rare, and their study can establish the theory of galaxy mergers in the early universe, which eventually results in a binary black hole system. Thus, this study, partially supported by the Polish Funding Agency, National Science Centre, can provide a better understanding of blazar OJ 287.