AstroSat View of Blazar OJ 287: A complete evolutionary cycle of HBL
  Component from end-phase to disappearance and Re-emergence
  We report three AstroSat observations of BL Lacertae object OJ 287. The three
observations caught it in very different flux states that are connected to
different broadband spectral states. These observations trace the source
spectral evolution from the end-phase of activity driven by a new, additional
HBL like emission component in 2017 to its complete disappearance in 2018 and
re-emergence in 2020. The 2017 observation shows a comparatively flatter
optical-UV and X-ray spectrum. Supplementing it with the simultaneous NuSTAR
monitoring indicates a hardening at the high-energy-end. The 2018 observation
shows a harder X-ray spectrum and a sharp decline or cutoff in the optical-UV
spectrum, revealed thanks to the Far-UV data from AstroSat. The brightest of
all, the 2020 observation shows a hardened optical-UV spectrum and an extremely
soft X-ray spectrum, constraining the low-energy peak of spectral energy
distribution at UV energies -- a characteristic of HBL blazars. The
contemporaneous MeV-GeV spectra from LAT show the well-known OJ 287 spectrum
during 2018 but a flatter spectrum during 2017 and a hardening above ~1 GeV
during 2020. Modeling broadband SEDs show that 2018 emission can be reproduced
with a one-zone leptonic model while 2017 and 2020 observations need a two-zone
model, with the additional zone emitting an HBL radiation.
