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

¹ Dipartimento di Fisica, Università di Milano, 20133 Milano, Italy
² ITESRE-CNR, 40129 Bologna, Italy
³ Osservatorio Astronomico di Trieste, 34131 Trieste, Italy (also ITESRE-CNR, 40129 Bologna, Italy)
⁴ Tuorla Observatory, 21500 Piikkio, Finland
⁵ Osservatorio Astronomico di Brera, 23807 Merate, Italy
⁶ Osservatorio Astronomico di Torino, 10025 Pino Torinese, Italy
⁷ Dipartimento di Scienze, Università dell'Insubria, 22100 Como, Italy

Corresponding author: E. Pian, pian@ts.astro.it

Received: 3 January 2001
Accepted: 8 June 2001

Abstract

Optical polarization of seven selected BL Lac objects in UBVRI bands was studied with the Nordic Optical Telescope from December 10-14, 1999. Two of them, 3C 66A and PKS 0735+178, were monitored for 4 nights for a total integration time of 4.75 and 5.5 hours, respectively. Other objects (1 Jy 0138-097, H 0414+009, PKS 0823-223, OJ287 and BL Lac) were observed sparsely during the run. Apart from PKS 0823-223 (more polarized than observed in the past), the sources show levels of flux and polarization consistent with results at previous epochs. 3C 66A and PKS 0735+178 were intensively observed during December 11 and 12 and exhibited variability of polarization, both on internight and intranight time scales. Wavelength dependence of polarization has been investigated, as well as circular polarization. The results are discussed within the standard model for BL Lacs.