Characterizing optical variability of OJ 287 in 2016 - 2017

TL;DR

This study presents a comprehensive multi-band optical and polarimetric observational campaign of blazar OJ 287 from 2016 to 2017, revealing variability in flux, color, polarization, and spectral index, and discusses potential physical mechanisms behind these phenomena.

Contribution

First extensive multi-band optical and polarimetric monitoring of OJ 287 over 15 months, providing detailed variability analysis and insights into physical processes.

Findings

No intraday variability detected in flux or color.

Multiple long-term flaring events observed.

Significant polarization swings and a bluer-when-brighter trend.

Abstract

We report on a recent multi-band optical photometric and polarimetric observational campaign of the blazar OJ 287 which was carried out during September 2016 -- December 2017. 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 multi-band (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.