Dense optical and near-infrared monitoring of CTA 102 during high state in 2012 with OISTER: Detection of intra-night "orphan polarized flux flare"

TL;DR

This study reports dense optical and near-infrared monitoring of CTA 102 during a high state in 2012, revealing an orphan polarized flux flare and intra-night flare, indicating highly ordered magnetic fields likely caused by transverse shocks in the jet.

Contribution

First detection of an orphan polarized flux flare in CTA 102, providing insights into magnetic field structure and shock propagation in blazar jets.

Findings

Detection of an orphan polarized flux flare with high polarization degree.

Observation of intra-night flare with similar total and polarized flux variations.

Polarization angles suggest highly ordered magnetic fields and shock activity.

Abstract

CTA 102, classified as a flat spectrum radio quasar at z=1.037, produced exceptionally bright optical flare in 2012 September. Following Fermi-LAT detection of enhanced gamma-ray activity, we densely monitored this source in the optical and near-infrared bands for the subsequent ten nights using twelve telescopes in Japan and South-Africa. On MJD 56197 (2012 September 27, 4-5 days after the peak of bright gamma-ray flare), polarized flux showed a transient increase, while total flux and polarization angle remained almost constant during the "orphan polarized-flux flare". We also detected an intra-night and prominent flare on MJD 56202. The total and polarized fluxes showed quite similar temporal variations, but PA again remained constant during the flare. Interestingly, the polarization angles during the two flares were significantly different from the jet direction. Emergence of a new emission component with high polarization degree (PD) up to 40% would be responsible for the observed two flares, and such a high PD indicates a presence of highly ordered magnetic field at the emission site. We discuss that the well-ordered magnetic field and even the observed directions of polarization angle which is grossly perpendicular to the jet are reasonably accounted for by transverse shock(s) propagating down the jet.