Exploring the nature of the 2016 γ-ray emission in the blazar 1749+096

Abstract

Recent Fermi-Large Area Telescope light curves indicate an active gamma-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 gamma-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 gamma-ray) plus very long baseline interferometry (VLBI) data to investigate the nature of the gamma-ray events. The gamma-ray outburst coincides with flux maxima at longer wavelengths. We find a spectral hardening of the gamma-ray photon index during the gamma-ray outburst. The photon index shows a transition from a softer-when-brighter to a harder-whenbrighter trend at around 1.8 x 10(-7) ph cm(-2)s(-1). We see indication that both the gamma-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 similar to 32 degrees, are detected about 12 days after the gamma-ray outburst. We conclude that both gamma-ray events are caused by the propagation of a disturbance in the mm-wave core.