Extraordinary Activity in the BL Lac Object OJ~287
  We use a continuous wavelet transform to analyze more than two decades of data for the BL~Lac object OJ~287 acquired as part of the UMRAO variability program. We find clear evidence for a persistent modulation of the total flux and polarization with period $\sim 1.66$ years, and for another signal that dominates activity in the 1980s with period $\sim 1.12$ years. The relationship between these two variations can be understood in terms of a `shock-in-jet' model, in which the longer time scale periodicity is associated with an otherwise quiescent jet, and the shorter time scale activity is associated with the passage of a shock; the different periodicities of these two components may reflect different internal conditions of the two flow domains, leading to different wave speeds, or different contractions of a single underlying periodicity, due to the different Doppler factors of the two flow components. We suggest that the modulation arises from a wave driven by some asymmetric disturbance close to the central engine. The periodic behavior in polarization exhibits excursions in $U$ which correspond to a direction $\sim 45^{\circ}$ from the VLBI jet axis. This behavior is not explained by the random walk in the $Q$-$U$ plane which is expected from models in which a pattern of randomly aligned magnetic field elements propagate across the visible portion of the flow, and suggests a small amplitude, cyclic variation in the flow direction in that part of the flow that dominates cm-wavelength emission. 
Philip A. Hughes, Hugh D. Aller and Margo F. Aller (University of   Michigan)