Optical polarization variations in the blazar PKS 1749+096

Abstract

We report on the variation in the optical polarization of the blazar PKS 1749+096 observed in 2008--2015. The degree of polarization (PD) tends to increase in short flares having a time-scale of a few days. The object favors a polarization angle (PA) of $40^\circ$--$50^\circ$ at the flare maxima, which is close to the position angle of the jet ($20^\circ$--$40^\circ$). Three clear polarization rotations were detected in the negative PA direction associated with flares. In addition, a rapid and large decrease in the PA was observed in the other two flares, while another two flares showed no large PA variation. The light curve maxima of the flares possibly tend to lag behind the PD maxima and color-index minima. The PA became $-50^\circ$ to $-20^\circ$ in the decay phase of active states, which is almost perpendicular to the jet position angle. We propose a scenario to explain these observational features, where transverse shocks propagate along curved trajectories. The favored PA at the flare maxima suggests that the observed variations were governed by the variations in the Doppler factor, $δ$. Based on this scenario, the minimum viewing angle of the source, $θ_\mathrm{min}=4.8^\circ$--$6.6^\circ$, and the location of the source, $Δr\gtrsim 0.1$pc, from the central black hole were estimated. In addition, the acceleration of electrons by the shock and synchrotron cooling would have a time-scale similar to that of the change in $δ$. The combined effect of the variation in $δ$ and acceleration/cooling of electrons is probably responsible for the observed diversity of the polarization variations in the flares.