2021MNRAS.507.3653M

Blazar variability can be described as flaring activities on a wide range of time-scales over a baseline flux level. It is important to detect and distinguish baseline flux changes from long-term flare variations, since the former may reflect state transitions caused by a secular change in bulk properties such as the bulk Lorentz factor or the viewing angle. We report such a transition observed in the 11 yr Fermi γ-ray light curve of the blazar 3C 66A, where the baseline flux of ∼1.8 x 10^–7 ph cm^–2 s^–1 persisted for three years and then changed over a month time-scale to ∼0.8 x 10^–7 ph cm^–2 s^–1 and remained in that level for the next eight years. Moreover, there is evidence for a similar shift in baseline flux in the optical band. Broad-band spectral energy distribution modelling for the two activity states reveals that the baseline flux change is consistent with an overall decrease in the Doppler factor, which in turn implies an increase in the jet inclination angle by ∼1° or a decrease in the bulk Lorentz factor by ∼25 per cent. We discuss the implication of such a variation occurring on a month time-scale.