An analysis is presented of the optical polarimetric and multicolour photometric ( BVRJ ) behaviour of the blazar PKS 2155–304 during an outburst in 2010 .
This flare develops over roughly 117 days , with a flux doubling time \tau \sim 11 days that increases from blue to red wavelengths .
The polarization angle is initially aligned with the jet axis but rotates by roughly 90 ^ { \circ } as the flare grows .
Two distinct states are evident at low and high fluxes .
Below 18 mJy , the polarization angle takes on a wide range of values , without any clear relation to the flux .
In contrast , there is a positive correlation between the polarization angle and flux above 18 mJy .
The polarization degree does not display a clear correlation with the flux .
We find that the photopolarimetric behaviour for the high flux state can be attributed to a variable component with a steady power-law spectral energy distribution and high optical polarization degree ( 13.3 % ) .
These properties are interpreted within the shock-in-jet model , which shows that the observed variability can be explained by a shock that is seen nearly edge-on .
Some parameters derived for the relativistic jet within the shock-in-jet model are : B = 0.06 G for the magnetic field , \delta = 22.3 for the Doppler factor and \Phi = 2.6 ^ { \circ } for the viewing angle .