An Optical-Near-Infrared Outburst with No Accompanying Gamma-Rays in the Blazar PKS 0208-512

TL;DR

This paper reports an unusual optical-near-infrared outburst in the blazar PKS 0208-512 that lacked an accompanying gamma-ray flare, suggesting complex jet physics and magnetic field changes.

Contribution

It presents the first detailed analysis of a gamma-ray absent OIR outburst in a blazar, proposing new explanations involving magnetic field changes or jet location variations.

Findings

OIR outburst without gamma-ray counterpart observed

Magnetic field change likely caused the OIR flare during interval 2

Jet location and Lorentz factor variations may explain the anomaly

Abstract

We report the discovery of an anomalous flare in a bright blazar, namely, PKS 0208-512, one of the targets of the Yale/SMARTS optical-near-infrared (OIR) monitoring program of Fermi blazars. We identify three intervals during which PKS 0208-512 undergoes outbursts at OIR wavelengths lasting for longer than 3 months. Its brightness increases and then decreases again by at least 1 magnitude in these intervals. In contrast, the source undergoes bright phases in GeV energies lasting for longer than 1 month during intervals 1 and 3 only. The OIR outburst during interval 2 is comparable in brightness and temporal extent to the OIR flares during intervals 1 and 3 which do have gamma-ray counterparts. By analyzing the gamma-ray, OIR, and supporting multi-wavelength variability data in details, we speculate that the OIR outburst during interval 2 was caused by a change in the magnetic field without any change in the total number of emitting electrons or Doppler factor of the emitting region. Alternatively, it is possible that the location of the outburst in the jet during interval 2 was closer to the black hole where the jet is more compact and the bulk Lorentz factor of the material in the jet is smaller. We also discuss the complex OIR spectral behavior during these three intervals.