Spectral States of OJ 287 blazar from Multi-wavelength Observations with
  $AstroSat$
  We present {\it AstroSat} soft X-ray, near-UV (NUV), and far-UV (FUV)
observations of a blazar, OJ~287, carried out in 2017, 2018, and 2020. The
simultaneous observations with NuSTAR in 2017 provide a broad-band look
encompassing NUV, FUV, soft and hard X-rays. Captured in three different
broadband spectral states in three observations, the X-ray spectrum is found to
be the hardest during 2018, while the high-energy-end of the simultaneous
optical-FUV spectrum shows a steepening that is modeled with a broken power-law
spectrum. The spectral energy distribution (SED) in 2017 shows a relatively
flatter optical-FUV and soft X-ray spectra, implying an additional emission
component. The 2020 optical-FUV spectrum is harder than in 2017 and 2018, with
an extremely soft X-ray spectrum and a hardening above $\sim$1 GeV, similar to
the SEDs of High-energy-peaked BL Lac objects (HBL), thereby establishing that
this additional emission component has HBL-like properties. The {\it AstroSat}
multi-wavelength observations trace the spectral evolution from the end-phase
of the HBL component in 2017 to its disappearance in 2018 followed by its
revival in 2020. A single zone leptonic model reproduces the 2018 broadband
spectrum while the 2017 and 2020 SEDs require an additional HBL-like emitting
zone. The spectral evolution of the high-energy-end of the optical-UV spectrum,
revealed by the FUV observations in 2017 and 2018, strongly suggests that X-ray
spectral changes in the normal broadband spectral state of OJ~287 are primarily
due to the evolution of the optical-UV synchrotron spectrum.
