The Long-Term Optical Variability of the BL Lacertae Object S5 0716+714: Evidence for a Precessing Jet

We present the historic light curve of the BL Lac object S5 0716+714,spanning the time interval from 1953 to 2003, built using Asiago archive platesand our recent CCD observations, together with literature data. The sourceshows an evident long term variability, over which well known short termvariations are superposed. In particular, in the period from 1961 to 1983 themean brightness of S5 0716+714 remained significantly fainter than thatobserved after 1994. Assuming a constant variation rate of the mean magnitudewe can estimate a value of about 0.11 magnitude/year. The simultaneousoccurrence of decreasing ejection velocities of superluminal moving componentsin the jet reported by Bach et al. (2005) suggests that both phenomena arerelated to the change of the direction of the jet to the line of sight fromabout 5 to 0.7 degrees for an approximately constant bulk Lorentz factor ofabout 12. A simple explanation is that of a precessing relativistic jet, whichshould presently be close to the smallest orientation angle. One can thereforeexpect in the next ten years a decrease of the mean brightness of about 1magnitude.Comment: to appear on The Astronomical Journal, 17 pages, 7 figures. Fig.2 is given as a separated jpg fil