The polarisation behaviour of OJ 287 viewed through radio, millimetre, and optical observations between 2015 and 2017

¹ Finnish Centre for Astronomy with ESO, University of Turku, Finland
² Department of Physics and Astronomy, University of Turku FI-20014, Finland
³ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, FI-02540 Kylmälä, Finland
⁴ Helmholtz centre Potsdam, GFZ German Research Centre for Geosciences, Germany
⁵ Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park IC2, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁶ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁷ Saint Petersburg State University, 7/9 Universitetskaya nab., 199034 St. Petersburg, Russia
⁸ Aalto University Department of Electronics and Nanoengineering, PO Box 15500 FI-00076 Aalto, Finland
⁹ Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

^⋆ Corresponding author; jesojo@utu.fi

Received: 15 November 2024
Accepted: 14 January 2025

Abstract

OJ 287 is a bright blazar with century-long observations, and one of the strongest candidates to host a supermassive black hole binary. Its polarisation behaviour between 2015 and 2017 (MJD 57300–58000) contains several interesting events that we re-contextualise in this study. We collected optical photometric and polarimetric data from several telescopes and obtained high-cadence light curves from this period. In the radio band, we collected millimetre-wavelength polarisation data from the AMAPOLA programme. We combined them with existing multi-frequency polarimetric radio results and the results of very long-baseline interferometry imaging with the Global mm-VLBI Array at 86 GHz. In December 2015, an optical flare was seen according to the general relativistic binary black hole model. We suggest that the overall activity near the accretion disk and the jet base during this time may be connected to the onset of a new moving component, K, seen in the jet in March 2017. With the additional optical data, we find a fast polarisation angle rotation of ∼210° coinciding with the December 2015 flare, hinting at a possible link between these events. Based on the 86 GHz images, we calculated a new speed of 0.12 mas/yr for K, which places it inside the core at the time of the 2015 flare. This speed also supports the scenario in which the passage of K through the quasi-stationary feature S1 could have been the trigger for the very high-energy gamma-ray flare of OJ 287 seen in February 2017. With the millimetre-polarisation data, we establish that these bands follow the centimetre-band data but show a difference during the time when K passes through S1. This indicates that the millimetre bands trace substructures of the jet that are still unresolved in the centimetre bands.