2018MNRAS.480.2324K

Recent Fermi-Large Area Telescope light curves indicate an active γ-ray state spanning about five months from 2016 June to 2016 October in the BL Lac object 1749+096 (OT 081). During this period, we find two notable γ-ray events: an exceptionally strong outburst followed by a significant enhancement (local peak). In this study, we analyze multi-waveband light curves (radio, optical, X-ray, and γ-ray) plus very long baseline interferometry (VLBI) data to investigate the nature of the γ-ray events. The γ-ray outburst coincides with flux maxima at longer wavelengths. We find a spectral hardening of the γ-ray photon index during the γ-ray outburst. The photon index shows a transition from a softer-when-brighter to a harder-when-brighter trend at around 1.8 x 10^–7 ph cm^–2 s^–1. We see indication that both the γ-ray outburst and the subsequent enhancement precede the propagation of a polarized knot in a region near the VLBI core. The highest polarized intensity, 230 mJy, and an electric vector position angle rotation, by ∼32°, are detected about 12 days after the γ-ray outburst. We conclude that both γ-ray events are caused by the propagation of a disturbance in the mm-wave core.