Multiband Optical Polarimetry of the BL Lacertae Object PKS 2155−304: Intranight and Long‐Term Variability

Abstract

The polarized and total Ñux of the BL Lac object PKS 2155[304 were monitored intensively and simultaneously in the optical UBV RI bands with the Turin photopolarimeter at the CASLEO 2.15 m telescope during four campaigns in 1998 June, August, and November and 1999 August. The e †ective observation time amounted to D47 hr. PKS 2155[304 showed a linear polarization percentage (P) usually ranging between 3% and 7% and a polarization position angle (P.A.) mainly between 70¡ and 120¡. The highest temporal resolution of our observations, 15 minutes, is unprecedented for polarimetric monitoring of this source and has allowed us to detect amplitude variations of the linear polarization percentage from 6% to 7.5% in timescales of hours. In some nights the polarization percentage seems to increase toward shorter wavelengths ; however, the polarized spectrum does not vary signiÐcantly with time. The most remarkable variability event occurred on 1998 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¡. This is consistent with the presence of two emission components, of di †erent polarization degree and position angle. Intranight variability of P and P.A. can be interpreted with small-amplitude physical or geometrical changes within the jet. Measurements of the circular polarization over time intervals of days set upper limits of 0.2%. Simultaneous photometry taken with the Turin Photopolarimeter and with a CCD camera at Astronomical Observatory did not show light variations correlated with those of the Co rdoba linearly polarized Ñux.

Multiband optical polarimetry of BL Lacertae objects with the Nordic Optical Telescope

“…This occurs also on an intranight scale and the overall behaviour is similar to that observed in B2 1308+326 (Puschell et al 1979), BL Lac (Moore et al 1987), S5 0716+174 (Impey et al 2000) and PKS 2155−304 (Tommasi et al 2001b). In the last three cases, total integration times were larger by an order of magnitude and more extreme variability episodes were recorded.…”

“…Five more BL Lacs included in the list were observed sparsely during the run: 1 Jy 0138−097 (only circular polarization), H 0414+009, PKS 0823−223, OJ 287, BL Lac. In Table 1 the list of the observed sources is reported along with some results and basic information (see following sections), together with three southern BL Lacs monitored by our team with the 2.15 m telescope of the Complejo Astronomico El Leoncito (CASLEO, Argentina; see Scaltriti et al 1999;Tommasi et al 2001aTommasi et al , 2001bTreves et al 1999).…”

“…e Doppler factor (Ghisellini et al 1998;Tagliaferri et al 2001). f Object observed by our team at the CASLEO telescope equipped with the Turin photopolarimeter (Scaltriti et al 1999;Tommasi et al 2001aTommasi et al , 2001bTreves et al 1999). strongly polarized BL Lacs .…”

“…The relativistic beaming amplifies the radiation from the nucleus with respect to the unpolarized light coming from the host galaxy and the polarization can be as high as 30%. Only a few objects have been observed up to now in search for polarization variability on intranight timescales (Puschell et al 1979;Moore et al 1987;Impey et al 2000;Tommasi et al 2001b). …”

“…Data presented in this paper have been collected with the Nordic Optical Telescope (NOT) during December 10-14, 1999, within a programme started some years ago and devoted to the optical polarization monitoring of BL Lac objects in both hemispheres, in a range of time scales from days down to less than an hour (Scaltriti et al 1999;Treves et al 1999;Tommasi et al 2001b).…”