Prominent polarized flares of the blazars AO 0235+164 and PKS 1510-089

We report on multi-band photopolarimetric observations of the blazars AO 0235+164 and PKS 1510-089. These two blazars became in active states in 2008 and 2009, respectively. In these active states, prominent short-term flares were detected in both objects, having amplitudes of > 1 mag within 10 days. The V - J color became bluer when the objects were brighter in these flares. On the other hand, the color of PKS 1510-089 exhibited a trend that it became redder when it was brighter, except for its prominent flare. This feature can be explained by the strong contribution of thermal emission from an accretion disk. The degree of polarization increased at the flares, and reached > 25 % at the maxima in AO 0235+164 and PKS 1510-089. We compare these flares with other short-term flares which were detected by our monitoring of 44 blazars. Those two flares had one of the largest variation amplitudes in both flux and degree of polarization. Furthermore, we found a significant positive correlation between the amplitudes of the flux and degree of polarization in the flares. It indicates that the short-term flares originate from the region where the magnetic field is aligned.