Multiband optical polarimetry of the BL Lacertae object PKS 2155-304: Intranight and long-term variability

The polarized and total flux of the BL Lac object PKS 2155-304 were monitor ed intensively and simultaneously in the optical UBV RI bands with the Turi n photopolarimeter at the CASLEO 2.15 m telescope during four campaigns in 1998 June, August, and November and 1999 August. The effective observation time amounted to similar to 47 hr. PKS 2155-304 showed a linear polarizatio n percentage (P) usually ranging between 3% and 7% and a polarization posit ion angle (P.A.) mainly between 70 degrees and 120 degrees. The highest tem poral resolution of our observations, 15 minutes, is unprecedented for pola rimetric monitoring of this source and has allowed us to detect amplitude v ariations of the linear polarization percentage from 6% to 7.5% in timescal es of hours. In some nights the polarization percentage seems to increase t oward shorter wavelengths; however, the polarized spectrum does not vary si gnificantly with time. The most remarkable variability event occurred on 19 98 June 18, when the degree of linear polarization decreased by more than a factor 2 in 1 day in all bands, while the P.A. rotated by 90 degrees. This is consistent with the presence of two emission components, of different p olarization degree and position angle. Intranight variability of P and P.A. can be interpreted with small-amplitude physical or geometrical changes wi thin the jet. Measurements of the circular polarization over time intervals of days set upper limits of 0.2%. Simultaneous photometry taken with the T urin Photopolarimeter and with a CCD camera at Astronomical Observatory did not show light variations correlated with those of the Cordoba linearly po larized flux.