Eight X-ray selected BL Lacertae objects were observed in the Johnson's BV and Cousins' R bands with the telescope of the Torino Astronomical Observatory from October 1996 to May 1997. In some cases (MS 0317.0+1834, 1ES 0347-121, and MS 0737.9+7441) variability on a time scale of a few days was detected.
Most of these objects have been seldom observed in the past, so that their optical behaviour is still largely unknown. The data presented in this paper represent a contribution to their study.
The observational periods include, or are close to, the BeppoSAX satellite pointings, so that the optical data can be compared to the X-ray ones and to others taken at different wavelengths in order to derive information on the emission processes that are at work in the BL Lacs jets.
Our data confirm that in general the X-ray selected BL Lacs do not present strong optical variability, in comparison with the radio selected ones. However, some of them have shown strong variability at X-ray energies. This kind of behaviour can find a possible interpretation as a purely geometric effect, without the need of an intrinsic flux variation. As shown in Villata et al. (1988b), one can assume that the synchrotron-emitting relativistic jet presents some kind of curvature (maybe with a helical shape); in this case only one (or some) part of the emission is strongly beamed towards us: the X-ray bright sources would be those whose X-ray emitting portion is more collimated with the line of sight with respect to lower-energy emitting parts. Small displacements of the jet would produce large variations of the X-ray flux, whereas the less-beamed optical radiation would not be affected by a corresponding variation. A detailed study of this scenario will be presented in Villata & Raiteri (1998).
AcknowledgementsThis research has made use of the NASA/ IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
Copyright The European Southern Observatory (ESO)