Blazars are one of the brightest sources in the universe. With the source of its optical flare, which was earlier understood to be a binary super massive black hole, a new study has found the source to be more complex and provides a better understanding of blazars and the physics powering the source of their optical flare, said a release from the Ministry of Science and Technology.

A special class of blazar is called 'BL Lacs', which shows rapid and large variability in emission. A blazar called 'OJ 287', whose central super massive 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 super massive black hole has been orbiting around the central black hole with an orbital period of nearly 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," the study said.

A group of scientists from Raman Research Institute, an autonomous institute of the Science and Technology department along with Rukayia Khatoon from Tezpur university, Bozena Czerny from Center for Theoretical Physics, Poland, and Pratik Majumdar from Saha Institute of Nuclear Institute studying the blazar OJ 287 -- who studied the second 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, current post-doctoral 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 the 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 behaviour of this source.

They found a significant change in the optical-UV and X-ray spectrum, which leads to a shift in 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, it said.

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 to high-flux state. The modelling 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, the release added.