Abstract:
We report on a recent multiband optical photometric and polarimetric observational campaign of the blazar OJ 287 that
was carried out during 2016 September–2017 December. We employed nine telescopes in Bulgaria, China, Georgia,
Japan, Serbia, Spain, and the United States. We collected over 1800 photometric image frames in BVRI bands and over
100 polarimetric measurements over ∼175 nights. In 11 nights with many quasi-simultaneous multiband (V, R, I)
observations, we did not detect any genuine intraday variability in flux or color. On longer timescales, multiple flaring
events were seen. Large changes in color with respect to time and in a color–magnitude diagram were seen, and while
only a weak systematic variability trend was noticed in color with respect to time, the color–magnitude diagram shows a
bluer-when-brighter trend. Large changes in the degree of polarization and substantial swings in the polarization angle
were detected. The fractional Stokes parameters of the polarization showed a systematic trend with time in the beginning
of these observations, followed by chaotic changes and then an apparently systematic variation at the end. These
polarization changes coincide with the detection and duration of the source at very high energies as seen by VERITAS.
The spectral index shows a systematic variation with time and V-band magnitude. We briefly discuss possible physical
mechanisms that could explain the observed flux, color, polarization, and spectral variability.
