Astron. Astrophys. Suppl. Ser. 133, 353-359

The variable sources responsible for the photometric behaviour of OJ 287 in the IR-optical-UV region during 1993-1996

V.A. Hagen-Thorn¹ - S.G. Marchenko¹ - L.O. Takalo² - A. Sillanpää² - T. Pursimo² - P. Boltwood³ - M. Kidger⁴ - J.N. Gonzalez-Perez⁴

1 - Astronomical Institute of St. Petersburg State University, 198904 St. Petersburg, Russia
2 - Tuorla Observatory, Tuorla 21500, Piikkiö, Finland
3 - Boltwood Observatory, 1655 Main St., Stittsville, Ontario, Canada
4 - Instituto de Astrofisica de Canarias, La Laguna, 38200 Tenerife, Spain

Received October 20, 1997; accepted June 22, 1998

Abstract:

An analysis is performed of multicolour observations of OJ 287 carried out during the OJ-94 Project. It is found that in the optical region the variable sources have the same power-law spectrum $(F_\nu\sim\nu^{-1.5})$ irrespective of flux level. In the IR the spectrum flattens during the flare possibly because of synchrotron self-absorption. The comparison of the outburst of 1994 with those of 1971 and 1983 shows that there is a clear correlation between the power of outburst and colour indices of the variable component in each event: the larger is the outburst, the bluer is the variable source, i.e. in more powerful outbursts the energy spectrum of relativistic electrons is flatter.

Key words: BL Lac objects; OJ 287 -- infrared: galaxies