Clues on the nature of the quasi-periodic optical outbursts of the blazar OJ 287

TL;DR

This paper investigates the quasi-periodic optical outbursts of blazar OJ 287, challenging the thermal bremsstrahlung origin hypothesis and proposing a synchrotron-dominated interpretation based on optical polarisation and spectral data.

Contribution

The study provides new insights by scrutinizing optical polarisation and spectral index data, arguing against bremsstrahlung and supporting a synchrotron origin for the outbursts.

Findings

Inconsistencies with bremsstrahlung interpretation are identified.

Optical polarisation data favor a synchrotron origin.

Radio non-detection during outbursts is explained.

Abstract

The fascination with the blazar OJ 287 stems not only from its status as a prominent candidate for a close supermassive black hole (SMBH) binary, but also because of the thermal bremsstrahlung origin proposed for its giant optical outbursts. These outbursts arrive as pairs, quasi-periodically every $\sim 12$ years, based on the unique 130-year-long, well-sampled optical light curve available for this blazar. For its three well-known, large quasi-periodic optical outbursts (QPOOs), observed in 1983, 2007 and 2015, optical photo-polarimetric monitoring has been reported in the literature. For these initially radio-undetected QPOOs, widely acclaimed as `bremsstrahlung flares', we have scrutinised the available measurements of optical polarisation and spectral index during the rising phase. Several inconsistencies of these data with the optical bremsstrahlung interpretation are noted, which point towards a synchrotron-dominated alternative interpretation for all these prominent QPOOs, just as for the optical emission observed between the outbursts. Possible reasons for the radio non-detection of the QPOOs during the initial stage are outlined.