Colour behaviour of the blazar PKS 0735+178 in 1994-2004 (Research Note)

Context. The properties of variable sources responsible for blazar activity are still under discussion. Some conclusions can be drawn from analyzing monitoring observations. Aims. The results of multicolour monitoring observations of the blazar PKS 0735+178 are used to study the colour behaviour of the variable source responsible for the blazar activity in 1994−2004. Methods. The method of “flux-flux” diagrams is used to find the SED of the variable source. Results. The SED of the variable source was unchanged on timescales of years and is represented well by a power-law. High observed polarization and power-law spectrum point to the synchrotron nature of the variable source. The achromatic variability can then be explained by variations in the Doppler boosting due to ∼10 ◦ -variations in the angle between the line of sight and the velocity direction of the radiating source. Conclusions. There is no doubt about the synchrotron nature of the variable source. The most probable reason for this variability is the change in the Doppler boosting.