| Abstract : | OJ 287 is a BL Lacertae object with emission dominated by a relativistic jet of plasma that is pointing almost toward us. It shows recurrent optical outbursts almost every 12-year and the most recent of these recurrent outbursts were accompanied by strong spectral changes across the entire electromagnetic spectrum that is untypical of this class of sources. At X-ray energies, it has shown strong spectral changes from very hard, intermediate, to extremely soft spectra and the intermediate X-ray spectrum has been argued to be a new emission component. We present a spectral detailed study of multi-wavelength observations OJ 287 with AstroSat that surprisingly caught the source in these three spectral states. The associated ultraviolet (UV) observations from AstroSat-UVIT during the low X-ray flux state show a cutoff at UV energies, supported also by the gamma-ray spectrum from Fermi-LAT during while no such cutoff is visible in UV during the intermediate state. Modelling the spectral changes with synchrotron and inverse Compton emission mechanisms using constraints provided by these three states, we show that the intermediate spectral state is primarily due to the evolution of the high-energy end of the synchrotron component. The contribution of the new emission component responsible for an extremely soft X-ray state is minimal. Combined with an indication of similar cutoff during the extremely soft X-ray state, our finding suggests that spectral cutoff is not due to radiative losses, as normally invoked in literature, but rather an outcome of the particle acceleration process i.e. the local physical conditions. |
