Tuorla Observatory, Tuorla, FIN-21500 Piikkiö, Finland
2 Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain
3 Department of Physics, Turku University, FIN-20014, Finland
4 Landessternwarte Heidelberg, Königstuhl, D-69117 Heidelberg, Germany
5 University of Oxford, Dept. of Astrophysics, Nuclear & Astrophysics laboratory, Keble Road, Oxford, OX1 3RH, UK
6 University of Michigan, Physics and Astronomy, 817 Dennison Building, Ann Arbor, MI 48109, U.S.A.
7 Hamburger Sternwarte, Hamburg Universität, Gojensbergweg 112, D-21029 Hamburg 80, Germany
8 Instituto de Astronomia-UNAM, Apto. Postal 70-264, 04510, Mexico, D.F. Mexico
9 1655 main St. Stittsville, Ont K2S 1N6, Canada
10 Osservatorio Astronomico di Torino, Strada Osservatorio 20, I-10025 Pino Torinese, Italy
11 Crimean Astrophysical Observatory, P/O Nauchny, 98409 Crimea, Ukraine, Isaak Newton Institute of Chile, Crimean Branch
12 Osservatorio Astronomica, Universitá di Perugia, I-06100 Perugia, Italy
13 Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate, Italy
14 Department of Astronomy, Indiana University, Swain West 319, Bloomington, IN 47405, U.S.A.
15 Metsähovi Radio Observatory, Metsähovintie 114, FIN-02450 Kylmälä, Finland
16 National Research Council Canada, Herzberg Institute of Astrophysics, Dominion Radio Astrophysical Observatory, P.O. Box 248, Penticton, BC, Canada V2A 6K3, Canada
17 Astrophysicalisches Institute Potsdam An der Sternwaret 16, D-14482 Potsdam, Germany
18 Instituto Astronomico, Universita di Roma “La Sapienza”, via G.M. Lancisi, 29, I-00161 Roma, Italy
19 Astronomical Institute of St. Petersburg State University, 198904 St. Petersburg, Russia
20 Department of Astronomy, Boston University, 725 Commonwealth Ave. Boston, MA 02215, U.S.A.
21 Abastumani Observatory, 38762 Abastumani, Republic of Georgia
22 The Astronomer Organization, Birmingham, UK
23 University of Colorado at Denver, P.O. Box 173364, Denver, CO 80217-3364, U.S.A.
24 Department of Physics, Florida International University, University Park, Miami, FL 33199, U.S.A.
Send offprint request to: T. Pursimo; e-mail: email@example.com
Accepted: 7 July 2000
We present intensive optical, infrared, and radio monitoring observations of the BL Lac object OJ 287, taken between the years 1993-1998. Two large optical outbursts were detected at the predicted times in November 1994 and December 1995. The detection of these outbursts supports the binary black hole model for OJ 287. Optical and radio polarisation observations show large variability in the degree of polarisation and position angle, very similar to those observed during the 1983/84 outburst in OJ 287. The polarisation position angles show very similar behaviour during these observations, indicating that, at least, the magnetic field orientations in radio and optical bands are related in OJ 287. Optical and infrared light curves show continuous variability in time scales ranging from tens of minutes to years. In the radio bands we have observed some of the lowest ever measured flux levels. During the first optical outburst in November 1994 the observed radio flux was very low, but during the second optical outburst radio bands also showed high flux levels. This is a puzzling observation, which can hopefully be used for discriminating between different outburst models. On top of the large outbursts OJ 287 has displayed flaring activity in time scales from days to weeks and shorter time scale flickering.
Key words: BL Lac objects / OJ 287 / photometry
Based partly on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway and Sweden in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofísica de Canarias.
Based partly on observations collected at the German-Spanish Astronomical Center, Calar Alto, operated by the Max-Planck-Institute für Astronomie, Heidelberg, jointly with the Spanish National Commission for Astronomy.
Based partly on observations carried out at Observatorio Astronomico National, San Pedro Martir, Baja California, Mexico.
© European Southern Observatory (ESO), 2000