Optical polarization variations in the blazar PKS 1749+096
  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, $\delta$. Based on this scenario, the minimum
viewing angle of the source, $\theta_\mathrm{min}=4.8^\circ$--$6.6^\circ$, and
the location of the source, $\Delta r\gtrsim 0.1$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 $\delta$. The combined effect of the variation in $\delta$ and
acceleration/cooling of electrons is probably responsible for the observed
diversity of the polarization variations in the flares.
